Could be bushing from the suspension. I think, not sure, your car has multi-link suspension? Also, check rear sway bar supports? Most of the time, bushing(especially factory) wear out either dry rot or what not and crack. So check all bushings and stuff. Also, make sure nothing is loose. For the bushings, you might want to upgrade to Polyeurathane bushings.
Also, one more thing, maybe you should also look at your exhaust. It might be loose too.