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.