After reading reviews from multiple people on this forum that have aftermarket exhausts from various manufacturers.. There is a common theme with drone in and around the same RPM range... This has me speculating that we are possibly hitting a resonant frequency of the car that is being excited by these exhaust systems.. My next inexpensive project is going to be to try to mitigate this by two things.. 1. Get some Soundskins damping and get a layer in the well and rear hatch area.. 2. Is to unbolt these and sandwich either a layer of damping or a rubber pad to further decouple the exhaust..