Got a bigblock 68 camaro convertible with a mild 454, stock converter and th-350 tranny. When the motor is warmed up, every once in a while with the car in drive I will get a distinct clunking sound under the floor boards that I can feel(through vibrations) under my feet. This is at idle. When I step on the gas it goes away and sometimes doesnt come back. What do you think it could be? Bad converter? u joint? tranny problem? Tranny mount? Any opinions would be greatly appreciated. Also, what steps can I take to diagnose this problem to narrow it down?

Thanks
Toofun

my car did the same thing and it was the exhaust pipes were run a little too close to the floor board and at idle they would vibrate against the floor board where they snaked the pipes up over the x-brace.

I appreciate the reply but not the exhaust pipes. It is more of a clunking sound like when you put a car in gear type of sound, not really a vibrating sound.

Toofun

Look at the exhaust really hard before you eliminate it as the source of your problem. The noises that they can make range from a loud clunk to a rattle. Also check the U-joints and the motor mounts. Good luck

also look at your subframe mounts and u joints. A bad ujoint will make that sound as well as a bad body bushing/subframe mount.

I just had my exhaust done, and I was a little too close to the floor. It gave similar symtons...occasional hard knock against the floor.

Check trans and motor mounts. If one is bad the engine could shift and your header flange or exhaust could hit the floor.

I don't have a convertible but I did have a real bad clunk or thump coming from the back of my car once in a while. It was my exhaust hitting my rear quarter on full comopression.I can't even tell you how many times I r&r the rear shocks and everyhting in the trunk. I finally found it by noticing a rub mark on the exhaust. Everyone i know thought it was a bushing or the spare tire moving around.