This is a common problem among 6-speed cars. GM is aware of the problem. The faint but distinct rattling noise can usually be heard with the tranny in neutral and the clutch out. A few quick revs at about 1/8 throttle and it should get louder for a fraction of a second, then disappear, and then return when the engine drops back to idle. You should also be able to hear it when starting out in first from 0-10 mph.

Unfortunately, there's no Technical Service Bulletin or recall to address this problem. But GM does know about it. In 1993 and 1994 cars, the GM Tech Assistant Group (TAN) called for the dealer to replace the clutch with a revised one. Two different part numbers for the revised clutch were found, (12551803 and 12551809), but no one is sure which is correct or if they really exist. 1995 and later cars are supposed to have a revised clutch (part #12551309), however, even people with these cars report to have same rattling noise. As of now, there seems to be no fix for the problem. It's even accepted to be normal by most dealers considering no one has reported any abnormal wear or damage which might be related to the noise.

from F-Body FAQ... LINK

 

Mine started making this noise after a cam install (kind of sounds like a diesel engine at idle) My mech. said it was the throw out bearing and the Cottman tranny guys said it was a bearing in the G/B. I had Cottman pull the tranny out and they said the bearings were fine so they put a new Luk clutch in it and gave it back it me with the noise still there. I started doing my own research and found that it is indeed gear lash noise and my new cam with it's loping idle causing it. If I bring the idle up to say 1000 rpm's it goes away. It doesn't do it when the car is cold either. I have read some posts saying that GM's GM Sycromesh gear oil will quiet it down. Has anyone tried using that?

 

I'm using it, and it didn't do a thing for quieting it down.

 

I recently did an A4 to M6 swap with a new McLeod clutch and flywheel. It made the exact same noise from the first time I drove the new clutch. It hasn't gotten any louder yet.

 

My trans has made the rattling noise since day 1, with the stock motor and with the 383, with the original clutch and with the new one.

I've just accepted it as a quirk.

 

I suppose I could live with it but one of my concerns is that it will make the car more difficult to sell when the time comes.........

 

odd. I know what you guys are talking about, I worked at the dealer for awhile. But for some reason my car doesn't do it...?

 

Shhhhh... you'll jinx it and it'll start making the noise!

 

mine never made any noise until the new cam went in. I think I would have stayed stock if I knew it was going to do that.......