While im driving.. sometimes, when I let off the gas.. I hear this strange knocking noise in my chasis somewhere.. it only happens when I let off the gas.. any light application of the gas pedal cancels it out.. this is very strange. It sounds like a woodpecker under my car..

 

perhaps part of the dash has come unclipped.......??......there was a recall on the stock exhaust because of a ratteling noise, but i doubt that applies to you......i would check to make sure all interor parts are clipped in, and that all ur susp. bolts are tight

 

Check your exhaust system. Might be anything from a bad or loose bolt, to a bad a-pipe (pipe that connects exhaust manifold to b-pipe under car). Might also be part of the exhaust system hitting the bottom of the car or something. Best bet would be to get under the car and check it out.

 

engine, tranny mount