Yes, Stethoscopes are cheap and very helpful.  Make sure you check areas all over the engine.  Such as the distributor and on the bellhousing.  Check the valve covers, A/C compressor, fuel pump, etc.

Thanks for all the suggestions fellas. Really busy with home renovations right now so not much time to work on the Vette. I've got an electrician coming on Monday and drywallers after that. We're renovating the living room and dining room, a bedroom and part of the kitchen. No money or time to play with the Vette.

Finally got some time to myself today. Got thinking about it being a timing issue as Joel suggested. I had put the vacuum advance back to ported a few days ago with no change; this time I disconected it altogether so now have no vacuum advance. Revved the engine back up and noise is not as bad...so it has to be knocking. I've never heard an engine knock with no load. Got the timing light out and found that the initial had drifted back up to 21 degrees from the 12 degrees I had it at it April even though the hold down was tight. Set the initial back to 12 degrees. Revved it up with the vacuum advance still diconected and knocking is all but gone. Checked the mechanical and I have 32 degrees all in at about 3000. Hooked the vacuum advance back up and it is knocking again. Disconect it and knocking is nearly gone. I did notice too that when I was trying to set the inital, the timing was wondering several degrees. What do you think, distributor? Timing chain? Cam? With the hold down tightened as much as possible I can wiggle the distributor a little bit. I took the car for a drive and I am still getting a bit of knock on the highway sometimes but not all the time like I was. Throttle reponse is pretty lazy now but I don't care about that. I guess the next step is to take a couple of more degrees out of the timing.


Update: Ordered a new distributor today. Rather than repair my old one I've decided to just replace it.

|UPDATED|7/21/2013 2:30:14 PM (AZT)|/UPDATED|

it sounds like an ignition timing problem to me. How many miles on the engine?

76,000 miles.

Have you taken a look at the balancer to see if the outside ring may be spinning on the inside. Many times the rubber will be drying out and that allows the ring to spin which makes the timing mark jump around. Hope its that simple! Rodney

Very real possibility Rodney but unfortunately I did check that out and it seems OK.

  How about the advance unit itself, or the weights in the distributor. One spring broke, one weight rusty and stuck, the activatior arm in the unit bent or hung up some how?

A stretched chain, worn timing gears, worn dist gear or bushings, worn advance weight posts....or just a timing light issue could all cause the mark to jump around. Even a corroded terminal inside the dist cap could do it. To check if the chain is stretched, you can remove the dist cap, and turn the engine by hand a little back & forth. If the rotor on the dist doesn't move until the crank has moved several degrees or more, the chain is stretched. It's normal to see a LITTLE looseness, but not more than a couple of degrees of crank rotation, if that much. But...if you can turn the crank an inch and the rotor doesn't move, there is too much slack in the chain, or the gears are worn, or both.

Worn dist bushings can allow the shaft to flop around, and the timing marks will jump when using a light, as can a worn gear on the dist, or even the drive teeth on the cam. Also, if the cam is moving front to rear, it can do it. The rear cam plug(in the block) could be pushed out a wee bit, or it might not have been installed all the way to begin with.

So many things can cause this issue, as far as the mark jumping around like that. If it is jumping around at 2500 rpms, I would be concerned, but if it is only jumping around at idle, not so much.

Well, new distributor has not arrived yet but in the mean time, took the car to a show about 25 miles out of town. Babied it all the way there. Noticed some concerning knocking at idle as I was pulling in to park. Started it back up about 8 hours later and knock was gone. Decided to try and make it home on the back roads keeping the speed to about 40 miles an hour. 10 miles out of town the engine made a quite a death rattle and a squeal and quit before I could reach the key. Got it towed home on a flatbed and it now sits in my garage. I guess it wasn't the distributor. Temp was about 190 just before this and oil pressure was running about 38 pounds; a bit lower than usual. My driving season is over for this year.