i am not sure what this is, but i am pretty sure its exhaust related. at around 2300k rpm if i am accelerating slowly, especially when the car is cold i get a vibrating sound coming from the rear. it sounds like its coming from above the seats but i am not really sure. my car has flows and an off road x pipe and i am wondering if this could be the cause as well. has anyone had this problem? is their a specific place that starts to rattle/vibrate while the car is in a certain rpm? any suggestions? thanks alot, its been getting really annoying.