Just took my car to the track yesterday and today I'm hearing some clunking at low speeds only. It feels smooth at highway speed so I'm inclined to rule out axles or gears being the problem. No whining or anything like that. Any ideas on what the problem could be? I'm going to jack up the rear end tomorrow and spin it by hand to see if I can observe the problem but is there any specific area I should be looking?

I was thinking possibly the driveshaft u-joint or maybe warped rotors from brake stands at the track. But I'm really no expert on rear end noises. Noise doesn't get worse or better while cornering and above 60 km/h it's not really noticeable.

 

Forgot to mention this is a late 80s 3.23 LSD 10-bolt; I think it's the Eaton??, 7.625" ring gear and 26 spline axles.

 

3rd gens didn't get the Eaton Posi, they got the Auburn or Gov Lock. Someone could have install the Eaton Posi, but if it's stock then it's probably the Auburn.

Brake stands are very hard on LSDs. If you watch the street cars while they are doing their burnouts you will notice that many times both wheels will start out spinning, but after a while one will stop spinning and the other one will spin faster. I don't know exactly what causes this, but it generates a HUGE amount of heat in the differential and can cook the oil. It's possible that this is what has happened to yours and changing the oil might help. In my experience the Auburn requires 2 bottles of the GM oil additive to make them stop popping when turning corners.

This might not have anything to do with your problem, so look around for other things before you pop the cover off. Check your transmission mount and other things to see if there is some damage somewhere else.

 

Ok it's the Auburn diff then. I just forgot what it was called.

It definitely doesn't get any worse when cornering, so I'm not sure whether or not that's a symptom of the 'cooked' oil or not. I will add however that I didn't realize it takes two bottles of LSD additive; I only used one when I filled the diff.

I took a brief look under the car and didn't notice anything obvious with the trans mount or driveshaft, but I won't have time to jack up the car until later. I would expect if the trans mount was broken though I would be able to feel it in the shifter.

 

From what I understand a clunking noise in the 3rd gen is a symptom of a broken transmission mount. It has to do with the way the torque arm goes in the trans mount.

If it isn't popping when you turn a corner then don't put in any more additive. The additive helps the cones or clutches slip smoother, but it can also make it unlock easier. If you don't need the additive then your differential will lock up better without it.

 

So are you saying maybe broken torque arm mount? I hadn't thought about that. I'll also add that I can feel the clunking with the trans in neutral as well. And I have solid engine mounts and a poly trans mount. I highly doubt the trans mount is damaged as I believe I would feel the extra slack but the torque arm mount is a possibility.

Anyway, thanks for the suggestions!

 

I should add that I can feel the clunking more than I can actually hear it.

 

Two ideas. First I would check your u joints. They have a tendency to clunk and can sound like its the rear. Second I would check the transmission. On my 99 Firebird I had a clunk at low speeds I swore it was my rear end ( zexel torson lsd). Then about two months later I was driving and right when my tranny was shifting from 1st to 2nd it clunked the last time. I lost all gears except 1st. I had to replace it the tranny. I dont want to scare you just my experience. Figured I would share.

 

Was your transmission an auto or manual? I highly doubt I could damage a T56 with a V6, but it's possible I guess.

I checked the driveshaft u-joints yesterday. No play that I can feel and the joints aren't siezed or anything. With the driveshaft removed and the car in gear, the transmission doesn't make any noises that I can hear.

I'm really hoping it's not the trans...

 

Sorry I didnt realize you had T56. I had an auto 4L60. So I highly doubt its your tranny. I wish you the best of luck.

 

Ever figure it out?

 

No. I'm waiting to take the rear end apart until I finish setting up my 9" and swap it out. I have a feeling though it's a twisted axle. I had a friend driving my car the other week and driving behind him it looked like the driver's side wheel had an ever so slight wobble to it. It's so slight you wouldn't actually notice it unless you were looking for something.

It's not really out of the realm of possibility. When whatever it is happened, I was at the track doing 2-step 3500 rpm clutch dumps under 9 or so PSI of boost, as well as long brakestands to heat up my slicks. I have a 6-puck with a high-clamp pressure plate so the shock to the drivetrain is instant.

In the meantime I'm just lightly daily driving it. It's not getting any worse, but I'm having a high-rpm misfire that I need to diagnose as well, so that's taking priority for now.