my rear seat belts were smacking the plastic on the sides and making a noise it took me a few weeks to finally realize what it was - and i get a squeek in the back too i think its something to do with the spare tire cover but i just got a louder exhaust to fix that
and ive had quite a few extremely loud car stereos in my day - heavy bass tears the crap out of a car - maybe im jsut getting old but i would never do that again - you can have a really great sounding stereo in your vehicle without all that junk anyways