Rear end "gronnnch" noise

JPhil

Huh?
Joined
Mar 26, 2008
Messages
1,361
Location
Loveland, Colorado, USA
When I take a hard turn, like a tight right accelerating around the corner of an intersection, I often get a "gronch" sound from the back. Or taking a tight roundabout at a bit of speed. Usually it is on a right turn, but lately I've heard it on tight left turn too. It sounds exactly like I would think a tire rubbing on fiberglass would sound. It's very short duration, never more than about a second.

It started over a year ago, only very occasionally. Now it is more frequent, maybe 25% of the time.

From my seat it sounds to me like it's coming from the passenger side rear, but perhaps that's just the way it sounds.

I have looked and looked and find no evidence of tire rubbing anywhere---fender, body, spring, T-arm, anywhere. No marks on the tires at all. I thought for a while maybe body movement on old worn out body mounts causing tire contact, but there are no marks anywhere of tire contact!

It occurred to me yesterday, maybe something in the diff or axles? I rebuilt the diff a few years ago, springless, with top quality parts & guidance from Mike Traccdog & Gary GTR1999. It performs flawlessly, no more banging on turns. I run 80W140 Redline synthetic gear oil for positractions and have never had to add a drop.

Dual mount spring, no sway bar. Bilstein shocks, about 7 or 8 years old. Rebuilt T-arms from Van Steel about 4 years old. Balanced axles with good u-joints, 4 years old. Diff cross member reinforcing plates with new rubbers. VBP OEM style calipers, 8 years old. Original rotors, good condition. New heim joint strut rods. Car lowered couple inches.

Tomorrow I will crawl under again and check tolerances for slop in axles and etc. I wouldn't think the stub axles would make this type of noise if the snap ring came off and I know the specs were damn near perfect on all that when I assembled it. It needs alignment, especially after I just replaced the strut rods, but it's not bad and this started before that.

I can't think of anything in the wheel bearings or calipers that would make a noise like that either, but I've been wrong once, no maybe twice, in my life before.

Anybody have any ideas I can check for?
Thanks

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I had something like that a long time ago. A hot rodder at work suggested replacing the diff oil and adding the gm posi additive. It worked, the sound was nasty.
 
What kind of oil did you have in it before? How old was it?
My diff used to 'bang' on sharp turns when I had regular gear oil in it with posi additive & spring loaded clutches. This Redline oil says "do not use posi additive."
I'm wondering if this has something to do with the side loading of the axles on cornering.
 
Obviously two separate cars, but my wife's '91 Miata had a strange grinding noise on a hard left turn.....I swore it was in the rear diffy.....turned out it was the right front hub.....the races had a burned up area in an area of the surface....maybe pull the front bearings just to have a good clean inspection, and then the rears....hate to mention that on the rear as I know it's a PIA.....


:eek:
 
Like Gene says, very much possible that you hear a noise from the front.....

But:

GM actually had a service bulletin for my 2006 C6: needed to swap the diff fluid to make that crunching noise go bye bye.... It was quiet noisy in turns..... I knew about the SB before I bought my '06 vert - pretty sure the seller did not know....lol

Anyways.... Replace the fluid with some conventional gear oil and the posi additive, I'd bet money it will fix it......
 
The guy was an avid hot rod type. He claimed GM additive was superior.

I'd only had the car a few years, don't know what it had before that.

Also had an e brake spring break and make terrible noises, but that was constant.
 
Like Gene says, very much possible that you hear a noise from the front.....

But:

GM actually had a service bulletin for my 2006 C6: needed to swap the diff fluid to make that crunching noise go bye bye.... It was quiet noisy in turns..... I knew about the SB before I bought my '06 vert - pretty sure the seller did not know....lol

Anyways.... Replace the fluid with some conventional gear oil and the posi additive, I'd bet money it will fix it......

Wasn't there something similar with c5 clutches sticking to the floor? Changing the oil was the fix.
 
OK, barring something weird like a front end bearing, I'm 95% sure it's just the diff clutches. I re-read the small print on the Redline gear lube bottle and it does say you can add posi additive if needed. Scott down at The Performance Center said he's had to do that sometimes on racing posi's set up real tight.
So I will pour some additive in and see if that takes care of it. Unfortunately I will have to wait to do that: It's f**king snowing again and I want to go drive the car immediately after I put it in to mix it up. If that doesn't do it, I'll change out the oil & see.

F**king snow! I swear there have only been about three weekends since the first of the year when the weather's been decent. But I'm only whining---I feel really sorry for you guys in the northeastern quadrant of the country!
 
Meh, been since Jan/Feb '97 since I seen a REAL LIFE flake of snow....

:smash::thumbs:
 
Get the alignment done first.

I never use synthetic diff lube but plenty do and without apparent issue. I use (2) bottles of the current GM posi additive and Lucas 85-140 gear oil.

When you tuned the posi was the cross shaft balanced?, meaning did the cross shaft slide freely in the case with the case in any position? Did you have any end play in the posi side gears?

You shouldn't have endplay more then 010 in the axles. The T/A spindles should not have more the 002 endplay and -0- lateral play. See if you can rock the rotors by hand at the 3 & 9 position. At 003 endplay you can move the spindle in/out by a wheel stud.

I had a tuned posi make whirling noise in corners, sounded like a saw blade vibrating, turned out the posi tune was correct but the GM additive that has been sold the last year or two must have changed. Once 2 bottles were added it was fine.
 
Gary,

When I did the diff I spent a long time getting all the tolerances as close to the small numbers as I could, the shaft was smooth sliding, the side gears preloaded and tight. I don't remember the actual numbers or details now years later, but I was pretty happy with myself how good it came out, well within all the specs you gave me.

Everything else felt good today. I have no numbers, but the feel was good on the spindles & u-joints & etc.

The alignment is becoming a real frustration for me---The steering box you rebuilt is so tight & responsive that now I can really feel the need for re-alignment, but the damn weather keeps screwing up my opportunities to get it on a rack. I was finally supposed to be able to get it in Saturday, all the stars were finally aligned right, and my friend at the shop came down with stomach flue on Friday! Maybe next week.....
 
Ive heard that noise a couple of times turning a tight angle up a steep driveway. Only a few times over the last decade. Diff handles power without complaining so I dont worry about it.
 
I dumped in a bottle of GM posi additive, and no more noise, just like that.

However, at the same time I did that, I put on my new wheels & tires. The fronts rubbed on the back lip of the fender at near full lock. The sound was almost identical! Just a little 'hollower' and raspy from the tread sipes. I could hear it was from the front and marks on the tires & fender proved it.
A couple minutes with a die grinder, and all is quiet again.

Yay for me! :thumbs:
 
Just read you solved the problem, good deal. Like I said I now recommend 2 bottles in a new rebuilt diff with tuned posi. Never had to use 2 before the new additive hit the market a couple of years ago.

I would still get the alignment done too.

So you like the feel of the blueprinted box compared to the "stock" version?

Enjoy the car.

Gary
 
Had some problems with my 255/60's hitting the finder in the front and the inner fender in the rear. I had to grind the front spoiler and in the rear I had to add a aluminum plate to the inner fender to prevent it from rubbing through the inner fender. I can tell you from experience it make a whole lot of racket when it rubbs + vibration.

I also tuned my posi and haven't had any problems so far. I used one bottle of additive on it. However I not sure if I have posi action with it. I still need to put one tire on tarmac and the other or sand or so and accelerate that way to see.

No whining no nothing, so all is good so far.
 
Just read you solved the problem, good deal. Like I said I now recommend 2 bottles in a new rebuilt diff with tuned posi. Never had to use 2 before the new additive hit the market a couple of years ago.

I would still get the alignment done too.

So you like the feel of the blueprinted box compared to the "stock" version?

Enjoy the car.

Gary

Those bottles of additive I have are probably 5 years old at least, so maybe it's the 'good stuff'. I still have one left.

That steering box you built is great--I have never driven anything before that feels so tight & responsive. With the old Flintstone wheels & tires it was still a handful to drive because the steering was so precise but the tires were shit, especially out of alignment. These new tires are much more predictable. The alignment is top priority now, so that I can start putting the new box & new tires through their paces!

I recommend to anyone & everyone--Let Gary build you a blueprinted steering box! You won't regret it, you'll be asking yourself why you waited so long!
 
I went the rack and pinion route, same difference..... Steering improved a lot.....
But: the tires do make a HUGE difference. Just push on the fender/ A-pillar sideways and watch the tire sidewall flex with these 255/60/15 tires.....it's ridiculous.... While I really like the look of meaty sidewall tires on these C3s I got some 17's wheels a few years ago, night and day difference.....
When you get the alignment done it's going to feel like it's running on rails..... :thumbs::thumbs:
 
I went the rack and pinion route, same difference..... Steering improved a lot.....
But: the tires do make a HUGE difference. Just push on the fender/ A-pillar sideways and watch the tire sidewall flex with these 255/60/15 tires.....it's ridiculous.... While I really like the look of meaty sidewall tires on these C3s I got some 17's wheels a few years ago, night and day difference.....
When you get the alignment done it's going to feel like it's running on rails..... :thumbs::thumbs:

My '87 had 16" wheels, and so upon getting my '72, I noted the crappy tires immediately....sold the OEM wheels/tires for ~300 bux, bought a set of '92 vette wheels/tires added VBP 2.5" thick adapters.....handled great for about 6 years, but 3.7 turns/ 16-1 ratio was just too lose for me, so I investigated a rack install by a fellow in Europe...Yoshi? and TT put me on to the Grand Am rack, I snagged one outta a junk yard for 35 bux...fresh rebuilt too.....:smash: 12-1 ratio.....:eek:
 
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