"somebody with 2 hammers hitting the bottom of the car" is exactly how mine sounded when my driveshaft U-joint self-destructed. Happened at odd times though. New driveshaft fixed it instantly. Mechanic I trust said needle bearing was loose in there. I was sure it was something about the right rear wheel but no.

Jalex, maybe install a new trans mount and new differential mount ?  If that doesn't fix it, it's OK, they were probably about shot anyway. So then do the expensive stuff if necessary.  Or if you can afford it, replace rubber at all eight points of contact per post above.

I got my car cheap because it was clunking. Thank you worn out trans mount !

Edited February 4, 20169 yr by Stanley

So I got a solid diff mount and it did for my series 1 240z. Clunk went down but still clunks due to play at the axle hubs at the wheels. And replaced both u-joints on the driveshaft since it had a bad u-joint. Runs smoother but still clunk due to axle hub play and still rattles hard when letting go of the gas going above 60mph like something hammering. Checked the driveshaft it has no marks on it as all that would indicate any rubbing or hitting anything. So I'm still wondering what's causing the rattling/hammering 

 IMO, If the rattling, hammering sound, at 60 mph, is more like a buzz, it is probably coming from the drive shaft. It spins about 3 1/2 times faster than the axles. Try to determine if the sound frequency matches axle or drive shaft speed.

At acceleration it's a vibrating. Let go of the gas it's like somebody with 2 hammers banging away at something like a drum roll. Think I need to replace all the bushings and such. 

I mentioned loose engine and transmission mounts.  One of the obvious signs there is if the gear shift lever is moving.  If it's solid, the problem's probably in the back.

 When I had driveshaft & u-joint problems. The vibration was much faster than a drum roll. Sounded like a 100 lb. bumblebee. The vibration was a buzz not even close to a drum roll. I agree with ZH. Time to look at the mounts. 

Yeah when I had dropped the rear end to do the solid mount I thought of replacing all the mounts and bushings. 

On serious deceleration the rear stabiliser was touching the tail-shaft on my race Z causing the 2 hammer sound.  On careful examination the stabiliser bar joints were cracked and the bar had pushed up close to the tail=shaft uni joint in front of the diff.

I figured out what the problem was.. Badly warped drums along with worn strut mounts and a blown shock. Yeah I still got some work to do.