Rear Deck cover
Haven't noticed any rattles from my XLE, but I added a mat across the rear deck to protect the plastic from the sun shining through that huge rear window. Wanted a thick fiber type to help isolate trunk and rear-tire noises somewhat, too. Found exactly what I was looking for - a cut-to-fit "replacement auto carpet" at Big Lots ($8.00, and you only need half of it). I wouldn't use this as carpet (it is basically an open-weave fiber mat with no backing), but it works well as a deck cover. It also reduces reflection off the deck when the sun is at a low angle behind.
The mat on top of the deck deadens high frequencies from the speakers a little bit, but it is hardly noticeable. I'll bet 1-2 layers underneath the deck would work well to help with the rattling problem, too.
One other thing I did to block trunk noise was to stuff a foam rubber block in the pass-through opening. This may have some effect on the sound, too, for all I know, reducing interference from the rear of the speakers.