SpiritOf76

OK, so I've noticed this a few times since I got the car, it seemed a bit more frequent today...
When I'm going VERY SLOW, like just barely rolling forward, there is a clunking noise coming from the rear. I notice it mostly when I'm just starting to move after being stopped, and usually when turning. For instance: I come to a red light, prepare to make a right on red, and inch forward to make sure traffic is clear, there is a significant and repeated "clunk, clunk, clunk" which can also be felt. It is not consistent. I even tried to get it to happen to show a friend what I meant, and of course it would not happen. On the way home from his house, of course it did it again.
Any ideas guys? Thanks.

speedreed8

front pinion rubber mount. Or ujoints to start with.

7D2

Sounds like the differential needs the GM limited slip posi additive.

Sluefoot

I agree with 7D2....Mine was doing the same thing, even after I had a trans. shop go thru it.....found out they did NOT put any diff. fluid in it....I added 1 bottle of 4 oz, then a 2nd when the 1st wasn't enuff.....been whisper quiet ever since!

dochorsepower

Agreed. Add posi fluid first. That's the quickest cheapest first step.

SpiritOf76

Okay, so I finally got around to having the rear diff serviced. New fluid plus the GL5 additive. Sound has not gone away.

blue by you

did you put in the gear fluid and the gm posi additive you can not use the gear fluid that says it has the additive in it

SpiritOf76

My shop did it, and yes they used the fluid and then put in the additive separate.

fake

Did you stand there and watch them? No? Go to another shop1 Did you get three estimates? Corvette repair is an open ticket to deception

SpiritOf76

Wow... that was unreasonably negative! No, I didn't stand there and watch them. Shop owner is a family friend we've been going to for decades, he did the job himself. Instead of unfoundedly assuming my shop ripped me off, are there any other legitimate possible causes for this noise? U joints? He also told me my radius arm bushings are shot. Could that be a cause or symptom? Helpful comments appreciated.

leadfoot4

OK, I know you have another thread going on, about the bushings. I replied to you there, and suggest you check it out.

Having said that, my wife had a '79 Corvette that she bought new, when we were first dating. We broke up shortly afterward, and didn't see each other for a number of years. Fate, and problems with her Corvette, that she couldn't HONESTLY get fixed, eventually brought us back together.

One of the issues she was experiencing with the car, was a "clunking" noise in the rear. I replaced the u-joints, as well as a dried-out differential snubber bushing. After addressing these potential sources of the clunk, it was still there.

Bottom line, after these fixes didn't work, I had to dig deeper, and wound up pulling the differential, where I found that the pinion shaft, that the spider gears rotate on, had worn the retaining holes in the differential carrier into an "egg shaped" pattern. Every time the car was accelerated, this shaft was "rocking" in the elongated hole, making the noise. I replaced the carrier, and the noise was gone.

Now I know that my wife didn't beat on the car, but unfortunately, she wanted to put the car away, for several winters, and she entrusted a "friend of a friend" to keep the car in his storage building. He'd take the car from her in November, and bring it back in March. I think, however, the car got a "hard ride" back an forth from storage, which caused this damage.

JayRay

Mine does the same thing, when I got under and looked the rear differential mounts were cracked, off. I have the new cover on its way so not sure if it is going to fix it yet. Take a look though. I just happened to notice it when doing a brake caliper change. Should be able to tell you in a week once I have the new cover on if it fixed the problem.

gjohnson

U Joints make a clunk when bad.That would be the next thing on my list to check

694speed350

SpiritOf76

it has it!!!

Big2Bird

There is a snubber bushing above the diffy at the front up top. See if its wasted.

dave14019

What about the limited slip clutches being too tight? My 75 makes the same noise when backing or turning at slow speed. Otherwise it's fine. Just had my master mechanic son rebuild the diff and replace all the u-joints. Used BG diff fluid with limited slip additive.

theandies

My vote:

DUB

the thick hockey puck looking one may look fine but the top one is where I have seen them fail mostly.

On an automatic car...you can get in the car and while HOLDING THE BRAKE with your foot...crank the engine and put the car in gear and have someone look at that area and then put it in reverse and then back in drive and watch that area. IF it is bad...they will see it move.

Also look at the rear transmission mount....along with the previously suggested Universal joints.

I have had few cars make noise and it be the rubber bushing at the lower shock mount area on the shock absorber itself.

LASTLY and it can happen and I have seen it also is that the rubber in your trailing arm bushing is shot and the entire trailing arm can move front to rear.

DUB

SpiritOf76

Taking it into the shop today... I will have them look at all these things.