My 2003 TL-S just started slightly squeeking when i go over the slightest bump. It sounds like the back seat or the rear deck but i know its not the sub. The car is out of warrenty so forget the stealership.

 

Well id suggest having some one drive teh car while you sit in the rear and try to figure it out.

 

Mine does the same thing...it sounds like its coming from the rear deck or up near the radio antenna where the little plastic box is mounted to the headliner. I got in the backseat yesterday and pushed around on some things (edge of headliner....and where the headliner joins to the interior plastic C pillar trim pieces) and drove the car later and it got a little quieter. The dealer wanted $100+ labor to remove the deck to investgate...I declined since they failed to fix the other rattles after I paid them for that.