thecobrahunter

iTrader: (1)

K,

So occasionally when I shift into reverse and start backing up, just as the tires start to move, I can hear this small clunk or thump from somewhere underneath the car from somewhere in between the seats. Only happens just as the wheels start to turn, and only sometimes, when I try to back up.

6speedt/a

iTrader: (1)

what mods do you have? If no suspension mods or headers start the the lca bushings and bolts to make sure they are tight and the torque arm bushing. if you have headers they are more that likely hitting on the floor or the k-member

thecobrahunter

iTrader: (1)

Well, have shorties and most of the the suspension upgraded. Was making the same noise some time ago before I bought a torque arm relocation brace (to keep my tailshaft intact). If I back up more slowly, no "thunk". If I back up a little faster, "thunk".

Formula1996

iTrader: (7)

I have a similar clunking noise happening on my car. I have the relocation brace for the torque arm as well with BMR LCA. I hear a clunk every time I transition from acceleration to braking (has to be more then modest acceleration). I tightened up the front torque arm bolts and that seemed to help for a little bit but then it came back. The other thing you might want to check is the transmission bushing, I've heard these can go bad and can cause a clunking noise as well. I haven't figured mine out yet but good luck with yours, and let us know what it was if you get it fixed!

ricks4genshianne

In my case, it was a destroyed tranny mount on the crossmember.

thecobrahunter

iTrader: (1)

K, well, everything is still intact underneath. Again, if I back out slowly, no thump. But (only) if i back up faster, that's when I hear it.

soulless

iTrader: (3)

I had the same issue when I installed my UMI t/a and relo crossmember. I went through every bolt in the suspensin with hand tools and it wouldn't go away. So, I took it to a friends shop to get the car in the air and hit all the bolts with air tools. Turned out to be the bolts for the arm/flange were a bit loose.

As for the OP, check trans mount?

eb110americana

iTrader: (4)

I was thinking another alternative could be driveline slop, like looseness in the U-joints or diff. If the noise happens just before the car starts moving, it could very well be this. If it happens just after the car starts moving, it's likely something more inertial such as exhaust contact or suspension.

Rednecksportscar

Ever figure it out? I'm having the exact same situation as the op

Chris Paveglio

iTrader: (2)

I had my exhaust moving around excessively. One joint is a little loose over the axle and would allow the rest to jiggle a bit. A couple times it got close and touched the driveshaft. So I had to fix that and put an additional (home made) hanger to keep in the right place.

wannafbody

iTrader: (2)

Could be headers and y pipe banging if you have headers. Otherwise, I'd suspect the torque arm bushing.

bad_408_vert

iTrader: (6)

Check your exhaust hangers. My car needs exhaust hangers and it rattles .

savage3070

Not sure this applies to your car.
Here is one that took me forever to find...... I have Viking rear coil overs that did not use the f body upper brackets. I was told you do not need the upper brackets - the body is strong enough to support the shock. I had the OEM style shock rods going thru the body with the OE style rubber bushing.. It works..... but, after spending many, many countless days looking for the clunk I finally found it was the top of the spring hitting the body anytime the suspension moved. You can not see up in there that well.but there is virtually no clearance..I changed the shock ends and added the top mounting brackets - problem solved. This brought the spring down and away from the body.

GMRL

For anybody that has adjustable rear lower control arms. If the jam nuts come loose, you will hear/feel a clunk right under the seats. Pretty close to where they bolt up to the body.