I'm hoping the collective wisdom of the forum can point me in the right direction with my problem.

I took my '77 out for a short test drive yesterday. This is the first time it's moved more than a few feet under its own power in 4 years (body off restoration to replace floor panels and address bad fiberglass repairs, and then "while I'm at" it syndrome ensued).

It runs, shifts and rides fine but when I let off the gas I get a noise from below/behind that I can only describe as a groan or low buzz. If I give it gas again, it disappears until the next time i let up on the gas pedal. While it's been a long time since I drove it, I don't recall ever hearing anything as obvious and worrisome before the teardown. Whatever the noise is, it's clearly not right and I don't want to do any damage by driving it until I know what's going on.

My first thought was rear end, but I don't want to dive into that unless i really have to. Putting it back up on stands and yanking the rear end would be a bitter pill since I just got it driving again.

Going back over my notes, here's the rear end related stuff I did:

1. Trailing arms, rear bearings, and half shafts were redone by DUB. Guessing this isn't my problem.
2. Replaced u-joints in driveshaft (front and rear).
3. Replaced pinion seal to fix a leak, but didn't go further into the diff. I didn't change bearings, but did put in a new crush sleeve. After reassembly I filled it up, and it's never leaked a drop since. I did not put in posi additive though - I just learned about that recently.

I've done u-joints in other cars and never had a problem with those before. I did my research before taking apart the yoke and pinion seal (made witness mark) and checked the preload for used bearings after reassembly. All seemed well.

Any thoughts? I'm really not sure how to diagnose this, or where to go from here. I have searched the forum, and the closest match to my symptom seems to be too little pinion preload. I'm not clear on how to confirm this or what to do about it, since I thought I had it set correctly on the bench.

Could it be something I haven't considered above?

Thanks in advance everyone.

 

My first thought was rear end

 

Pinion bearings or bad ring gear setup Since you didn't take the diff apart or install new gears it's most likely the pinion bearings. They are not correctly set back to the preload prior to the seal change or the bearings were damaged.

 

Hows your fluid level....did you add 2 bottles of of the small posi additive? Is it when you are pulling up to a light or when you down shift and engine brake from a high speed or high RPM?

 

Don't know if this has anything to do with your issue but not having posi additive in a posi rear end is not a good thing. 2 bottles are recommended.

DC

 

Drive it a while to see if 'things' free up. Might be a little rust in some 'stuff'. If it sounds like gear noise, it probably is. Could it be a leaky exhaust manifold or exhaust joint?

 

You can and should try adding 2 bottles of the gm additive but usually that shows up as chatter taking a turn. However, if it is posi chatter and resolves it that is good fix but most times the noise you describe letting off the gas and hearing it is not posi chatter but gear whine or pinion bearings.

 

Thanks everyone for the fast responses.

@7T1vette , I don’t think it’s exhaust. I tried coasting in neutral and revving the engine - the noise didn’t change.

@DC3 I’m going to put in additive before I take it out again. Might hep, surely can’t hurt.

@Rescue Rogers , this was just a short run around the block. I never got over 40. Can’t recall for sure but i believe it happened in all gears (it’s an auto). I can check again after I put in posi additive.

@GTR1999 , is there a way to know if it’s too loose or too tight? If loose, can I tighten the pinion nut a smidge until the noise disappears? Praying I didn’t jack up the pinion bearings.

 

If the pinion was too loose as soon as you applied load, as in moving forward, the pinion would crash into the posi case. It would be a loud metal on metal noise. If the pinion was too tight the bearings will be wrecked. I don't know what method you used replacing the pinion seal but here's how I do them. I wrote that 14 years ago to help guys out.

(1) 63-79 Pinion Seal Replacement | Corvette Forum : DigitalCorvettes.com Corvette Forums

 

OMG, I groan when I sit down. I groan when I get up. I hate to think what I'd do if I tried to decelerate.

 

I recently replaced the pinion seal on my 73 Following GTR1999's above linked tutorial (thanks Gary). I did it on jack stands with the rear end in the car.
At first i used the old nut and just tightened it back to the witness marks that i had made before removal also counting the turns. I was afraid to overtighten the nut. After a couple of short drives, I was Having the same issue. A whining on deceleration. After jacking the car back up and removing the drive shaft again, I found that the nut had backed off a little so it wasn't tight.
I replaced the nut with a new one and just tightened it down good. This was early June. I've had no further Issues with it.

 

I used your guide when I replaced the seal - that’s the research I alluded to above 🙂.


​​@Finally173 - interesting info. While I’m under there putting in additive I will have to drop the shaft and check the nut tightness. It may have backed off like yours. Thanks!

 

What about the snubber and the bolts. If those are loose its going to make noise when you decelerate

 

I will check that too. And I’m hoping it’s that simple. But the snubber bushing was replaced (every piece of rubber on the car was rotten so all were changed) and I torqued everything to spec so I’m pretty certain that’s ok.

 

The nut needs a drop of #271 loctite and I stake them as well. If you didn't do that then it might backed off. If you didn't hear metal on metal thats good as you might just need to snug it. If you hold the yoke with a pipe wrench you should be able to feel when the bearing snugs up to the sleeve.

 

I got rid of the groan in mine. Leave your wife at home.

 

Dalton....
I got a kick out of your post. But my problem is that I can't ACcelerate !!!

 

You might already know to do this but just in case - find a place nearby after putting in the two bottles of additive and do several tight figure 8s with the car to work the additive in.

DC

 

I had the same thing in my 72 a while back, GTR1999 is right, pinion nut not torqued to spec

 

Nope, thanks for the tip. It's now up on stands so I can get the driveshaft dropped this weekend and utilize all the advice I've been given. Man, I wish I had a lift!
Figure 8s will have to wait a few more days.