My 04 TL's subwoofer in the started becoming distorted with a rattling sound three to four months after I purchased the car. It eventually fried completely. The dealership replaced it under warranty, but within a few days after replacement, I began hearing a rattling noise from the rear again. Has anyone else had this particular problem? The level on the sub is only +1, and this sound can be heard with the volume at or around 20. Seems strange for such a quality automobile.

 

On mine, after 9 months I started to ear a king of rattle coming from the rear deck where is the subwoofer. When I push on the deck, the rattle disapear.

So I realize that the rear deck was not properly in place, near the rear seats. I had to push the deck in place, near the rear seat in place. It probably went down about half centimeter and the rattle disapear.

 

I have removed the deck lid and removed the sub and put foam tape under the speaker, the stuff that's there is really thin.
Also while back there I noticed the center seat belt retractor in kind of a cheap design. The retractor is held on to the thin rear metal deck with ONE bolt.
I've found this rattles a lot. I just tweaked it some and the rattle stopped.
Maybe this might help some of you with rear deck rattles.