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Topic: rat tat tat noise, right rear

in Forum: C3 Fuel, Emission Control, and Exhaust Systems

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rat tat tat noise, right rear (1/27)
4/27/14 2:07pm

JonsVette
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Eldridge, IA - USA

Vette(s):
Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

Joined: 4/26/2014
Posts: 28

Hello to all, here is my first post for help. I posted in the fuel section because I am leaning toward the fuel pump being the source. Anyway, I have bought a low mile 78 Coupe. After filling the tank up and driving it about 150 miles home, there was no issue except typical anti freeze drips from the hose edges etc....The next morning, after some clamp tightening and what not, I took it out for a spin and after drive 10 miles, returned home. As I was pulling into the garage, low RPM, I heard a rat tat tat noise from the right rear. I did not hear this as I was driving the speed limits. At idle, in the garage, the noise continued. So, I blocked the car and got out to inspect. I narrowed it down to the right rear wheel area. As the noise continued, I banged on the catalytic converter (thinking something had broken loose inside the converter) but it didnt change the sound. Baffled, I put my fingers on the fuel lines that sit on top of the frame in the right wheel well. The pulsation of the rat tat tat sound could be felt. So I proceeded to take the car out for another spin to trouble shoot this. Again at high RPM, I didnt hear the noise. Upon finding a flat stretch of road, I pushed the clutch in and let the engine idle down and I could hear the rat tat tat noise again. I shut the engine off and coasted down the road. No rat tat tat noise. So, and Thanks for bearing with me, I am leaning toward the fuel pump being the culprit. If I am off base, please input your thoughts. I really appreciate it. Jon

|UPDATED|4/27/2014 11:07:45 AM (AZT)|/UPDATED|

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Re: rat tat tat noise, right rear (2/27)
4/27/14 12:52pm

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Wichita, KS - USA

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Jon, you might check your exhaust pipe and muffler for a loose clamp or baffle in the muffler. I had that happen to mine and had a rattle like you describe. Good luck.

Larry

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Re: rat tat tat noise, right rear (3/27)
4/27/14 2:16pm

JonsVette
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Eldridge, IA - USA

Vette(s):
Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Larry, I appreciate the response. I had checked all exhaust connections and nothing found. If you can imagine taking a hammer and hitting a 1 inch steel pipe, that is what the sound is similar to. A constant rapping, or rat tat tat, that doesnt stop unless I shut off the engine. When the engine is off and the clutch is in, there is no noise. So I ruled out rear end issues and parking brake hang ups etc.... I first thought I had lost a valve but the drive train/valve train is solid and quiet. When the noise happens, I can feel the pulse of the "tat tats" on the fuel lines laying on top of the rear frame where it arches over the rear axle area. My assumption is that the fuel pump may be failing inside. Just not 100 percent on that theory. It drives me nuts at a stop as you can hear it loud and clear. As soon as the rpms go up, the sound disappears or at least goes faint and absorbs itself within the roar of the exhaust and road noise. Jon

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Re: rat tat tat noise, right rear (4/27)
4/27/14 2:46pm

johnu
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What does it sound like up on the lower right side of the engine block where the fuel pump is? The rat-tat-tat could be the short rod that drives the pump. I replaced my pump last summer. Mine was spraying gas out the vent hole....removal-replacement was not the most fun I've had.

|UPDATED|4/27/2014 11:46:45 AM (AZT)|/UPDATED|

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Re: rat tat tat noise, right rear (5/27)
4/27/14 3:03pm

JonsVette
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Eldridge, IA - USA

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Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Thanks for the response. When checking it out, it was louder on the back end of the car. My first thought after I felt the fuel lines was the same - the fuel pump was giving it up - and since the pump is not in a great spot, I didnt get a chance to put a hand on the housing. But I am certain I felt the percussion of the tat tat on those fuel lines, or something was knocking against the lines somewhere on the frame bottom. I did look from the top side to see if he pump mounting bolts were loose or coming loose but all was tight against the block. Ugh. I will try to troubleshoot it more. Thanks. Jon

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Re: rat tat tat noise, right rear (6/27)
4/27/14 7:02pm

dyoes
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Zachary, LA - USA

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Take a long piece of metal (crowbar or similar) and rest it on the fuel pump, then to your ear. I doubt the fuel pump internals are making that much of a noise, but it may be the pump pushrod that's driven by the engine cam. If you need a new fuel pump, PM me... I have a brand new stock Corvette pump (with the built in vapor return) that I'll make you a hell of a deal on.

I bought an Edelbrock pump and was convinced it wasn't working so I bought another stock one. Turns out you have to have enough fuel in the tank for ANY fuel pump to work, and once I put enough gas in the tank the Edelbrock pump worked fine. Embarrassed

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Re: rat tat tat noise, right rear (7/27)
4/27/14 7:36pm

JonsVette
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Eldridge, IA - USA

Vette(s):
Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Thanks for the reply. I actually was trying to find my Texas screwdriver (about 3ft long) to do just what you said, put it against the pump, but I will be damn if I can find it. (its hard to lose) I appreciate the offer and will contact you about the fuel pump when I get a chance to find that screwdriver and then fire up the Vette and do further diagnosis. I guess when I am thinking the internals are failing- it also means the fuel pump rod maybe part of that equation as well. I dunno. Been working on cars a long time, never came across this scenerio before. Jon

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Re: rat tat tat noise, right rear (8/27)
4/28/14 1:03pm

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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I'd find it hard to believe a fuel pump noise would travel all the way back to the rear of the car.....the pump is connected to the fuel lines with rubber hoses. The only way a noise could travel like that would be if the hoses are rock hard....in which case ya need to change them anyway! A fuel pump can certainly make a rat-a-tat noise like that, either from an internal failure, OR the pump rod wearing....I've seen/heard both....but never from the rear of the car. If the pump or pump rod is creating the noise, you would be able to hear it at the front, and you could put your hand/fingers on the pump and feel it.
Check those hoses!
Another thought is that possibly the sock filter on the pick-up tube on the fuel sender is collapsed, or otherwise damaged. If so, it could allow the fuel pump to create pulses back there that would possibly transfer to the line by the tire, but it would not indicate a problem with the pump itself.
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|UPDATED|4/28/2014 10:03:33 AM (AZT)|/UPDATED|

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Re: rat tat tat noise, right rear (9/27)
4/28/14 2:25pm

JonsVette
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Eldridge, IA - USA

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Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Joel, excellent response. That is an angle I didnt think about. Yes, the rubber hoses are there of course so maybe checking the fuel tank area is a more viable option. I was double thinkin g my routine from that day and it seems that this noise started after the tank got to quarter level. Up to that point it was quiet. Not sure if theres a connection there or not. Anyway, great response and some more fuel for the thought process, I really appreciate the help. Jon

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Re: rat tat tat noise, right rear (10/27)
4/28/14 2:28pm

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Duncanville, TX - USA

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#1-1974 L-48 4spd Cp Med Red Metallic/Black deluxe int w/AC/tilt/tele./p/w-p/b/ Am-Fm/map light National/Regional/Chapter NCRS "Top Flight" #2-1985 Bright Red/Carmine Cp.L-98/auto Member: NCRS, NCRS Texas, Corvette Legends of Texas

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I believe the '78 up cars also had some sort of roll-over valve in the tank, since the fuel sender comes out the top. Could be something with that thing, too. Try loosening the fuel cap while the noise is happening, and see if that makes any difference....might lead us to a different solution.

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Re: rat tat tat noise, right rear (11/27)
4/28/14 8:15pm

JonsVette
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Eldridge, IA - USA

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Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Joel, Thanks again for responding. I will pull the sending unit out. I believe I can pull the unit out from the fill cap area on this year of Vette, so at least I wont have to drop the tank. I will verify that procedure before I begin- as the memory isnt as it once was. Jon

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Re: rat tat tat noise, right rear (12/27)
4/28/14 9:43pm

dyoes
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Zachary, LA - USA

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1974 L-48 Auto. Just your basic Vette. GM crate motor w/vortec heads and intake. Crane cam. Scorpion rockers, Edelbrock carb. Lots of parts still in boxes... :(

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Adams' Apple said: I'd find it hard to believe a fuel pump noise would travel all the way back to the rear of the car.....the pump is connected to the fuel lines with rubber hoses.

Dang, you're right. Didn't think about the rubber hoses.

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Re: rat tat tat noise, right rear (13/27)
5/3/14 12:17pm

JonsVette
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Eldridge, IA - USA

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I will let everyone know what I find out on this issue. I am setting aside some time on Sunday to check into it. If there are any other ideas or suggestions, please pass them on. Thanks Jon

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Re: rat tat tat noise, right rear (14/27)
5/4/14 6:02pm

JonsVette
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Eldridge, IA - USA

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Sunday May 4th.......OK, here is the story. Ran the vette for half an hour. Gas level at 1/4 tank. After reaching operating temp, ran high idle (55mph equal) for 1-2 minutes. After idle went to curb, rat a tat a tat started up again in the right rear. Put the long screwdriver to the surface of the fuel pump and no abnormal noise at all. Sounded good. Removed the fuel cap and issue did not change. I pressed the screwdriver on the fuel lines riding on top of rear frame arch and positively pinpointed the sound. Plain as day, rat a tat tat , loud and clear. Lines running toward bottom where the rear dog leg meets the straight run of the frame were quiet. So, something is affecting the fuel lines from appx a 2-3 ft run from the fuel tank until they make the slope down the right rear dog leg. Supply line was cold to the touch and a fuel return line was allowing a steady stream of fuel back to the tank. The only two things I can think of are- as previously mentioned, something collapsed with the fuel sock (I looked in the tank through the fuel fill hole but all looked normal) or some kind of line shock is happening or combo of both. The only time this has done it was when the fuel level got this low. When I bought the car and drove it home, there was 3/4 of a tank of fuel and never had the noise. Drove it a couple of more times before this noise started appearing which was just about the 3/8th area of the fuel gauge. So, at my wits end. Any ideas? ? Thanks Jon

ADDENDUM- Decided to just remove the fuel sending unit and replace it. For the cost its worth eliminating that as a possibility. Was hoping to keep the car with as many of the original parts but sometimes it just wont happen that way. Thanks for the suggestions, will let you know if the problem is solved......

|UPDATED|5/4/2014 3:02:33 PM (AZT)|/UPDATED|

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Re: rat tat tat noise, right rear--update (15/27)
5/12/14 1:08pm

JonsVette
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Eldridge, IA - USA

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Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Ok everyone, need some opinion input. I think I have narrowed down what is causing this noise. It has to be air getting into the line somehow. Since the car is bone stock original, I am assuming that maybe the rubber hoses are dried up and the clamps are weakening. I dont believe it is the sending unit, fuel pump or fuel pump drive rod. I believe it is time to replace all the original rubber connection hoses and the original clamps. I hate to do this as I want the car as original as it was from day one, but I like driving my cars so- its a love hate thing. Any suggestions are much appreciated. Thanks Jon

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Re: rat tat tat noise, right rear (16/27)
5/12/14 2:20pm

cingraham
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Brookline, NH - USA

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see if you have brake dust covers and see if they are hitting the brake caliper mine does this and it hits in certain areas only !! I pushed on the the cover a little bit were you can see scratch marks on the cover where it hits the caliper very little clearance and it made a rat tat noise that was driving me crazy !!! from the right rear. Sometimes it's just something stupid it fixed the noise but cam back a month later had to push on the covers again and noise is gone !!

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Re: rat tat tat noise, right rear (17/27)
5/12/14 3:31pm

JonsVette
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Eldridge, IA - USA

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Posts: 28

Thank for the response. I dont have caliper covers. I had pinpointed the noise at the rear area of fuel lines. After meeting up with a couple of mechanic buddies this past Saturday, we all came to the conclusion that air maybe getting into the fuel line somehow. The noise and the pulsation action felt on the line is equal to getting air trapped in a water line. Anyway, since this past Sunday was a "no work in the garage day" I will tackle the issue this week. Any other ideas are appreciated. If I am off my rocker on this, let me know ! It is one of those issues that doesnt happen often enough to generate any history from someone. Thanks again. Jon

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Re: rat tat tat noise, right rear (18/27)
5/12/14 4:14pm

dskopp
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Oak Creek, WI - USA

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1981 Great White Shark. Red Interior, 350/190hp. PS, PB (SS), A/C CC, T-Tops, Going to remain as Stock as possible. Served three years in Active Duty Army, then Retired Air Force after 34 years! Badger State Vetts Car Club. 175,000 Original miles!!

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JonsVette said: Thank for the response. I dont have caliper covers. I had pinpointed the noise at the rear area of fuel lines. After meeting up with a couple of mechanic buddies this past Saturday, we all came to the conclusion that air maybe getting into the fuel line somehow. The noise and the pulsation action felt on the line is equal to getting air trapped in a water line. Anyway, since this past Sunday was a "no work in the garage day" I will tackle the issue this week. Any other ideas are appreciated. If I am off my rocker on this, let me know ! It is one of those issues that doesnt happen often enough to generate any history from someone. Thanks again. Jon

Ya, Jon I got one of those "mystery" problems also. Mine is that the drivers window does not want to go up and down sometimes. I have checked the window switch and I have followed all the wires through the door and to the motor but I cannot find a problem. I have power at the switch and also checked for continuity. JOEL!!! HELP!!

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Re: rat tat tat noise, right rear (19/27)
5/12/14 7:05pm

rraider1
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Woodland, WA - USA

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possibly the grounding of the motor itself?

Re: rat tat tat noise, right rear (20/27)
5/12/14 7:34pm

JonsVette
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Eldridge, IA - USA

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Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Hi, electrical issues are a pain in the arss. Anyway, I would agree with the other poster about the ground issue with the window motor. Maybe get a replacement motor and trial run it and see if the issue develops again. There maybe something internally with the motor that acts up under certain conditions. Good luck. Jon

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Re: rat tat tat noise, right rear (21/27)
5/12/14 8:00pm

dskopp
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Oak Creek, WI - USA

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1981 Great White Shark. Red Interior, 350/190hp. PS, PB (SS), A/C CC, T-Tops, Going to remain as Stock as possible. Served three years in Active Duty Army, then Retired Air Force after 34 years! Badger State Vetts Car Club. 175,000 Original miles!!

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rraider1 said:

possibly the grounding of the motor itself?

Seems to me that if it was a moto grounding problem, it would either work or not work!! I can be sitting still and try the window switch and it will not work. Then I can wait for a couple of minutes and it will work!!! W@TF?????

Dan da Man

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Re: rat tat tat noise, right rear (22/27)
5/13/14 6:22am

JonsVette
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Eldridge, IA - USA

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Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Assuming you have tested the switch for weak contacts and checked the wiring for any breaks, then it would default to the motor assembly. I have seen motors with "weak spots" or in other words, internally something is going out or has a weakness. I have even seen a new replacement motor work fine on bench testing then upon installation into the door cavity and after hooking up the regulator and window it went weak and failed. I figured it was the weight and the force of resistance after hooking up the motor. I installed another replacement and it worked fine and sent the faulty motor in for warranty. Anyway,I would still just opt to put a new motor in and see what happens. Process of elimination. Of course it could be the dreaded electrical gremlins. Jon

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Re: rat tat tat noise, right rear (23/27)
5/13/14 12:19pm

dskopp
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Oak Creek, WI - USA

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1981 Great White Shark. Red Interior, 350/190hp. PS, PB (SS), A/C CC, T-Tops, Going to remain as Stock as possible. Served three years in Active Duty Army, then Retired Air Force after 34 years! Badger State Vetts Car Club. 175,000 Original miles!!

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Ya, replacement of the motor is the next thing for me to do....after the gathering. Did you fix yopur rata-tat-tat?? I see you are on the site now!!

Dan

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Re: rat tat tat noise, right rear (24/27)
5/13/14 7:01pm

JonsVette
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Eldridge, IA - USA

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Posts: 28

Hi, I started last night by working on the rubber lines off the fuel pump. Next will be to fire it up and see if the noise is gone. If not, then onto the tank area where I will replace the rubber lines there. (I see there is some kind of bracket on the feed line right off the sending unit of the tank. Not sure how to remove that to access the rubber line if I need to. Any suggestions?) So will see what happens. ....All the females of the house are leaving Friday night till Sunday afternoon so it will be Steely Dan on the garage stereo and lots of Vette tinker time ! Yes, I joined the registry as a standard member and will probably go life member sometime soon, since the C3 is my favorite Vette generation. (But that 66 convertible I had was nice too.) Jon

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Re: rat tat tat noise, right rear (25/27)
5/18/14 11:07am

JonsVette
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Eldridge, IA - USA

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Ok, here is the rat tat tat update. After having Steely Dans' album finish up and somewhere in the middle of Bob Seger, my buddy and I came to the conclusion that air is definitely getting trapped in the fuel line back by the tank. Instead of spending a lot of time (which we had wasted already on beverages and song) I mentioned that maybe plugging the return line off the fuel pump could address the issue. So, on Sat. that is what I did. After driving the 78 almost 50 miles in various speed zones, the rat tat tat noise has gone away. Even though I havent repaired the cause, at least the cause has been determined. So the next step is to replace all the rubber connections on the fuel lines and tank and hook it back up to see what happens. But for now, I am keeping the line plugged because it is too nice out to work on the car, I need to go for a couple of drives. Anyway, there you have it, if this happens to your vehicle, I hope this thread helps in your diagnosing the issue. Jon

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Re: rat tat tat noise, right rear (26/27)
5/18/14 9:21pm

dwright
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Victor, NY - USA

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Jon,

When you replaced the sending unit and sock did it look OK inside the tank?

Was the bladder OK?

Did it sound any different with the new parts?

Does the sound go away when the rpms go up and you are not moving?

Did you try it with a full tank yet?

Does it idle smooth and still make the sound?

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Re: rat tat tat noise, right rear (27/27)
5/19/14 8:10pm

JonsVette
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Eldridge, IA - USA

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Have owned a 1966 Convertible, 1974 Convertible, 1978 Coupe, 1988 35th Anniversary, 1997 Coupe. All have gone on to new owners.

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Posts: 28

Hi, to answer your questions......

When you replaced the sending unit and sock did it look OK inside the tank? I didnt get to that part yet as I only plugged the return line. I did purchase a sending unit though as I was heading that direction.

Was the bladder OK? From what I can see through the fill hole it appears clean but again I didnt remove the sending unit yet.

Did it sound any different with the new parts? Not to that point yet.

Does the sound go away when the rpms go up and you are not moving? The sound did disappear on high rpm, 1500-2500 but reappeared at curb idle.

Did you try it with a full tank yet? I did fill the tank prior to disconnecting the return line and the problem remained.

Does it idle smooth and still make the sound? There was never any idle or performance issues with the rpms. The car would idle fine even with the noise going on.

Anyway, so far that is all I got done. I drove the Vette a bunch of miles on both Sat & Sunday with no noise reappearing.

I seen where you had some brake issues returning home. I would bet the master cylinder failed. Anyway, fine piece of driving !

Thanks Jon

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