when i accelerate i hear a rattling kinda like a pipe hitting something gently until the rpms get higher and i cant hear it anymore. it does this 3/4 through 1st and 1/4-1/2 through second. anyone have any ideas on what it could be. i sounds more like its coming from directly under the car, not the engine compartment.

 

Mine used to do that as well, and I took it to an exhaust shop. They just said the welding on it was horrible and they re-did it for me. Rattling ended.