Pontiac GTO Forum banner

awful scraping noise from rear

4K views 19 replies 8 participants last post by  Stereolab42 
#1 ·
No, it's not the busted shock from my other thread. I lopped off the offending brake bracket and replaced the shocks. It's something else. Only appears to happen when turning right over deep and wide ruts in the street:



I have 5 inches of clearance on everything under the car, and don't see any scrapes, so it's not that. It doesn't quite sound like gears grinding so I hope it's not the diff, although correct me if I'm wrong. Hell if I can figure out what is rubbing against what underneath there.
 
#4 ·
Let's assume for the sake of argument the diff is toast and I need to replace it. (My shop will tell me for sure on Thursday.) Car was born with a Safe-T-Track and AFAICT it's still on there (even has the metal tag). Let's also say I don't want to wait weeks to send it out for a specialized rebuild and just want to swap something in there ASAP that works. What do I order? I know absolutely nothing about diffs and have been furiously Googling around for an hour... looks like I need a diff carrier and diff gears at a minimum?

What about these 3.36 gears:

https://www.summitracing.com/parts/yga-24503/overview/

What about this Auburn unit (is the 28 splines correct):

https://www.summitracing.com/parts/aub-5420113/overview

Do I need "bearings" as well, and what kind? Anything else? (Once again, like Rummie, I feel like I don't even know what I don't know. :(
 
#6 ·
I agree with PJ, the best is to have someone outside and listen, video diagnose is hard at times, you can get a bunch of different opinions. If you just did a 4 wheel disc brake conversion, and the noise wasn’t there with the old set up and you know this, to me as I listen it sounds metal to metal, then my money is on the conversion, you might have a rotor rubbing on a caliper or a dust cover, etc. to me ( and I’m not there ) doesn’t sound internal, and I could be wrong, just offering advice. Good Luck
 
#7 ·
That sound is VERY familiar. Go check for evidence of tire rubbing on the REAR/LOWER corners of the FRONT fender openings... You'll also see rub marks on the tires themselves, right around the outside edges of the tread. See the location I've indicated in the attached photo. (Which, by the way, is a photo of my car that the original owner sent me from when it was brand new. :) )

Bear
 

Attachments

#8 ·
I can confirm that my '65 has rubbing in this exact spot (drivers front tire at back of fender) when I am driving downhill (even a small incline) and turning to the right as weight and gravity do their things. Same goes for the opposite (passenger front tire inside of fender) when turning the opposite direction on the downhill. I don't have the issue going uphill as the airshocks on back seems to keep everything happy.

The plan to fix for me is a front-end rebuild as the rubber/bushings/springs/shots have all had better days (years) as well as replace the airshocks/springs on back to keep everything in sync.
 
#11 ·
For what it's worth, is it scraping or "clunking" ? My 68 GTO, which had been restored a couple of years earlier, was doing the same thing, but in the front. Took me forever to figure it out. Mine only did it when I turned also so I though I'd add my two cents. It ended up being lose body mounts. I tightened all of them and it repaired the problem. Maybe it's worth a check.

Larry
 
#12 ·
Definitely a scraping, which implies a moving piece is impacting against a non-moving piece. Since I can't see any marks at all on the wheels/tires or surrounding hardware, and nothing on the lowest points of the car which have plenty of ground clearance anyways (5+ inches everywhere), the mystery remains. Maybe an axle in one of the rear axle tubes is scraping (assuming it's not the diff itself)? I know they had to remove/modify/reinsert the rear axles when doing the full disc brake kit. Luckily I'm taking it back to the same shop tomorrow so they are already very familiar with this problem child.
 
#13 ·
Odd that it only does it when you turn however. Does the scraping stop when you straighten out? That was my issue. I though I had tires that were too wide or the wrong length shocks. Ended up being the lost body mounts and the body was shifting on the frame. Let us know what you find out. GOOD LUCK.
 
#16 ·
Took it to the shop... had me drive up and down a dip turning right while he crawled underneath... says it's the driveshaft impacting against the muffler. Apparently the car was bending a bit when turning over a dip which was causing that contact. I looked for myself and do see where the end of the driveshaft is very close to the right muffler, and there is indeed a gouge there. So the muffler just has to be shifted.

Lesson for today is that clearance between different components under the car should be enough to account for torsion and bending.
 
#20 ·
The story gets even better... took it in today for the shop to actually fix the scraping, which was accomplished quickly. I mentioned that in addition to the scraping there is a groaning noise when I brake below 10 mph. They found the cause of that too... the upper rear control arms had been installed on the wrong sides! Instead of stopping to consider whether he had them swapped, the PO just went "hold my beer" and whaled them into place, bending the arms, the bolts, and the bushings in the process. $300 later for all-new parts and this will be fixed too. Yay!
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top