vibration problem help

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.
 
I know exactly what you are talking about. I have the same exact issue, although mine is not all the time. I got under the car and tightened everything with my cat-back and the sound was still noticable arouns 2k-3k. I think that what we are hearing is the wing on the back of the car. I think I am going to remove mine to see if it stops, plus I like the look of a wingless GT.