I have a 95 firebird formula with 90k and an auto. I have a clunk that shows up mostly when I'm slowing down and the trannies downshifting. It does it under power also but its not so much a clunk as a tiny amount of slack being taken up. Its quiet enough that most folks would prolly ignore it...but I'm enough of a mechanic to know I need to find out what it is before it strands me. Its def something in the drivetrain, and not something rattling or loose under the car. I had the fuel pump replaced by the dealership at 85k and had them check everything to find the clunk. They said all they could find was a loose tranny tailshaft bushing....they replaced it but clunks still there. They had the whole back of the car off twice and didn't seem to find anything wrong I know the tranny mounts break...but they said it was fine. I'm strongly suspecting I have a sloppy rear-end. The rear end made some noise on right or left turns when I first got the car, but a differential oil change with a limited slip additive cured that. It makes a very tiny whine now...but so slight it almost sounds normal for this car. I reprogrammed the ecu with a hypertech programmer for much harder shifts...and I run her hard sometimes, and nothings changed. No bigger clunking...no noises..nothing The cars had this noise since I got it..but it hasn't changed abit over 6k miles of highway trips and stoplight drags. Overall the cars runs and handles awesome...man these LT1's have some giddy-up-go in them Did I mention its fast!!! lol I just don't know whats normal noises/clunks for this car and what to check. The cars in great shape/runs like a top, but I suspect the previous owner had a pretty heavy foot kinda why I suspect a sloppy rear diff.
My check engine light will also come on for maybe 20-30 seconds then go back off almost everytime I drive the car...its not always at the same time and only does it once...whether I'm running to the store or taking an 8 hour road trip up or down in elevation. Any ideas on if that is normal or something to worry about? Sorry for the long first post. Any feedback will be greatly appreciated

 

Check out the torque arm, may be loose in the mount- that arm torques up on acell, and down on braking-

Could be free play in the rear end gears, too-

I am assuming that you checked the u-joints!

I get a power-on/power-off clunk, too, in my auto- almost think it is something in the output side of the tranny- been doing it for about 25,000 miles, still waiting for it to break so I can do a performance rebuild on the tranny- ;^)

 

Yep ***** I'm starting to think its just the nature of these cars. I'm gonna jack the car up myself and look, but its had the clunk since I got it, and its had the tank out/back end apart twice for fuel pump/dealer screw up (they broke the neck off the original tank putting it back in ) and they looked it over trying to find the clunk....they replaced the tranny tailshaft bushing and seal...it was leaking (which I knew about) and they said it was loose. It seemed to help the clunk some but its still there. It only does it almost coming to a stop then accelerating, or at parking lot speeds when I just barely get to the 1st to 2nd shift point...otherwise it never makes a sound under full throttle sideways shifts or regular driving. I'll post up what I find when I get under the car tho
I'm starting to suspect its the slow speed shift programming in the tranny thats just making the normal driveline lash sound worse.