1981 4 speed manual car. Driving along 40mph, hit clutch and drop to neutral, slight popping coming from the rear of the car. Lightly hitting brakes makes it noticibly launder, release brakes and noise goes away, brake again noise does not return. Do not hear anything on acceleration, but between shifts hear same light popping. Don't really feel this, just hear it, sounds like exhaust popping except for the braking thing..... Comment on what you think is going on, I will get it up on jack stands tomorrow and take a look, U joints are my first assumption but why does it get louder under light braking and then go away? Why don't I hear or feel anything when I accelerate? What else should I be looking for? Trailing arms replaced by PO, professionally done. Unfortunately they did not do anything with rubber bushings so these do need attention, probably original to car. Drivers side half shaft seal is leaking, not too bad but enough that there is a light spray of oil on underbody where it's slinging off the yoke.

Once you get the rear of the car up in the air also check the half shaft flanges at the trailing arms. I chased a noise similar to yours for months. One day I had the one half shaft out and checked to see if the flange at the trailing arm was tight. The nut was tight alright but the splines where worn. If that is the case with your car, they do make them still. I put both sides in and no more noise. 

Can you elaborate on that a little, you mean where the whole assembly mounts to the frame? Where the diff bolts to the aluminum cross member? I'm not sure what you are referring to when you say "differential mount". Is there a single point mount at the yoke to a crossmember? Is that what you are talking about? I can't imagine you could change the two outboard mounts at the frame rails in an hour!

|UPDATED|4/10/2016 11:24:36 AM (AZT)|/UPDATED|

OK, so the car is up in the air on jack stands, suspension on the rear is thus fully extended resting on what I'll call the "stop bolts" just behind the trailing arm pivot bearings. In this condition, when I grab the tire at the 9:00 and 3:00 positions and try to wiggle it there is zero movement. However when I grab it at 12 and 6 I can wiggle it up and down maybe 1/8 to 1/4" as measured at the top of the tire. The trailing arm is moving, twisting around the pivot point so to speak. Is this movement normal? Before I jacked it up I tried pushing on the top of the tire with my foot, rocking things side to side to see if I hear any noise or see any movement, it seems solid. The half shaft U joints are all fine, look to be new with the trailing arms, no lash and no side to side movement. The drive shaft U joint at the rear seems to have a tiny tiny amount of play in it, it might be my imagination, it's so hard to get up there and feel it. Lash through the ring and pinion does not appear to be excessive, just a little slop there. In short everything about the driveline seems to be in decent shape. Rolling the tires around by hand yields no pop, snap etc. I'm stumped, this up and down twisting of the trailing arm bothers me though. Again the rubber bushings everywhere are original and in awful shape, I guess maybe start there? I am a little mystified as to what keeps the trailing arm from rotating in the direction I'm moving the tire, there is the lower tie bar connected below the axle but what about the top side? Are the shims in the trailing arm meant to be tighter or something?

The front carrier mount is viewed in the two attached photos. You can normally reach up to the top side of the thru bolt and feel a gap between the bolt head and washer if the bushings have excessive wear. The mount I am refering to is the red puck in the first photo. It just a matter of unbolting the bracket from the diff. And replacing the mount. About $35.

|UPDATED|4/10/2016 12:55:17 PM (AZT)|/UPDATED|

That single mount up front is pretty rough, but I it's intact. With the car on the jack stands I started it up and ran through the gears a bit, just up to about 30mph, nothing..... I would think I would hear something. Went reverse to forward to reverse up and down in each gear, outside of a little tire shake absolutely nothin. Maybe I'm imagining the noise.....

You are more than likley NOT going to get your noise to appear with the rear tires off the ground. The drive train needs the resistance of the tires on the ground under de-acceleration torque. If you say the front mount looks pretty rough why not install a new one and see what happens? The new mount kit is inexpensive, can be change in a short time and it's a starting point. Just my 2 cents.

|UPDATED|4/10/2016 3:19:08 PM (AZT)|/UPDATED|

Yeah I think I will.

if you look at the picture of the diff unit on the floor, you'll see two bolts close to each other where the metal bracket bolts to the diff housing. Check these bolts, they will loosen up over time. I also had a noise inr the back end as well. What I found on Deceleration sounds like your noise. The bolts loosen just enough to allow some movement, which is modified through the frame and body to sound pretty bad. I tightened these bolts and my noise stopped. Granted, the nose bushing MAY be bad as well. Replace it! It is probably due. ALL small movement is exagerated as other thing add to the problem. Keep us in the loop as to what you find.