Mine only does it for the base; depends on the song. I don't listen to hip-hop so if you do that could be the problem. But whenever there's a normal bass note it will rattle and it's annoying.
I imagine simple foam to cushion/absorb it would work. I haven't expereinced the rattling in the rear but I did read about others that have.
Some say take it back to the dealer, others say its just part of owning a subbie.
My only other issue is the USB hook up. Sometimes it will just stop playing. The dash indicates the song continues to play but there's no sound.
To fix this I have to switch to blue tooth (I use a iphone 4 which has blue tooth ipod streaming) -- disconnect, reconnect the cable and then switch back to USB on the aux button. This works.
Good luck! Let me know what you do.
i saw your post about your 2011 WRX, and im experiencing similar problems with mine. my front right speaker rattles and buzzes pretty much all of the time. does yours also suffer from rattling in the back near the trunk?