I have a 94 z28. At normal running temp I can hear a slight knocking noise coming from the floor board area. Either in gear or in park. This is driving me nuts!! If I am at a light I can hear it, and as soon as I accelerate it goes away. If I open the hood it sounds like it's coming from the area above the passenger wheel well. This noise is not present when the engine is cold, only at normal operating temp. Please help!

 

Check all your exhaust clearences, flanges, and hangers. i had a simialiar prob, rattle and knocking at idle and certain RPMs, all at normal operateing temp. My exhaust was loose where the cat meets the I pipe.

 

It was the motor mounts on mine. Sound sounded like it was coming from front fender. Just go under and retighten the mounts.

 

Thanks for your replies. Turns out it was the tranny. Needs a rebuild.