I have an '04 GTO with 140,000 miles. Whenever I accelerate with the engine speed at 2700 to 3500 RPM, I get this rapid thumping that can be heard and felt--so much so during hard throttle that it feels like something is pounding on the vehicle. It is more prevalent in the higher gears and only does it during acceleration/deceleration. I've had it in the shop twice for this but the source has not yet been found. Has anyone else had this problem?