Could be 2 things (among others). More than likely, your rear most exhaust hanger ripped and the exhaust is bouncing around. Hopefully your's hasn't gotten as bad as mine did and melted part of your bumper. Just put your foot on it and see if you can push it to the side much.
The hanger was fine until I ran some Koni's at full firm in the rear for a couple track and autoX days. Fortunately its a $8 fix.
The second could be that you need the P51 shims. The upper rear shocks have about 1/2" of play at the top, and especially with a stiffer-than-stock suspension, can bang around a bit. Definitely worth the $20 or whatever P51 is charging and should be something everyone with a stiffer-than-stock suspension gets. Just do a search for the "p51 shims" over in the suspension forum.