the descriptions you guys are giving sounds exactly the same as to the sound my car is making, but i know how mine happens, some guy rewelded the back of the exhaust (rusty i presume) but anyway its on the ever slight angle so that it touches on the back bolt of the leaf spring, if i put rubber in between there no rumble, or if i pull the exhaust to the right of the car, and yes it only happens once revving high and letting the revs come down, its damn weird, the rubbers i put in between keep falling out so she rumbles..i love it