This is a question on my '98 Trans Am (M6) with the wimpy 7.5" f-body rear, thought I'd post it here too since there is so much rear axle expertise here:

My stock 3.42 rear with barely 20K miles on it started making a strange noise when coasting the other day. Its a clicking / knocking sound that is in sync w/ the driveshaft. You can't hear it when accelerating, or driving at a steady speed above 20 MPH, but as I slow to a stop (clutch in), I hear it. Thinking it might be the U-joint, I raised the rear axle off the ground and checked it out. The noise is definitely coming from the rear end and you can only hear it when the drive shaft is rotated in reverse or the rear tires are rotated forward (similar forces on the gears as when coasting). Would a chipped pinion gear cause this noise? Or maybe a pinion bearing? I can't detect any abnormal play in the pinion shaft (I didn't disconnect the driveshaft to check this). Could a bent torque arm change the pinion angle and cause this? Any advice would be appreciated.

 

You will need to crack it open. Your just guessing any other way... Is the pinion nut good? Meaning does the pinion have any in out play at all? Even the slightest amount is not good. only twisting is aloud NO IN OUT.

If you did break a tooth off...Most of the time you do not even notice.... I have seen many come in with a tooth snapped for overheating and just pop off clean and never affect the rear when driving. lol. I ask if they did not hear a noise from the rear and they would 99% say ..nope smooth and quiet...lol.
I am sure there are many ways to break a rear so best to do a physical visual inspection since it seems like you have done all the external inspection and found nothing you can detect for the issue.

Jim

 

Thanks for the feedback. I was planning on taking it apart to confirm if there were any broken pieces, just need to buy some fluid so I can put it back together in case I don't find anything wrong there. I suspect it is also best to disconnect the driveshaft to get a good check on any play in the pinion. Fortunately, this isn't my DD either, so its not going to kill me if its out of commission for awhile. If I do find something bad, this will be a good excuse to upgrade to a 12-bolt or a 9" and get rid of this wimpy 7.5" rear. I'm not pushing a bunch of power (lid and a tune, completely stock LS1 & exhaust otherwise), but I've not been gentle (nor abusive) on it either. I just didn't want to sink $3K or so into a rear axle for this car this summer. I have other plans for my Olds with that money.

Speaking of noises, I had begun to hear some driveshaft speed related noises earlier this week (normally only hear the gear whine). I remember hearing a pop when I hit 2nd gear pretty hard coming home for lunch yesterday, and after that this driveshaft noise seemed to be a little louder. I had thought the pop was something bouncing around in the trunk or tire / suspension bounce. I better brace myself for the worse when I take that cover off.

 

Based on those rears being critically weak, you may have chipped a gear.
The pinion diameter is rather small & are prone to deflection that will remove pinion teeth.

A bearing usually howls instead of a click or knock.
A u-joint going out can also cause a sound you are describing.

It is not uncommon to blow these up with a stock engine & manual trans.
Been there & done that a bunch of times.

This is what you need... a Strange S60.

 

Indeed, I've been checking those out too. Thanks.

 

I noticed the broken teeth when my spider gears exploded. It sounded like going over the railroad tracks.