I took my 2011 Acura TL SH-AWD in for the PCM transmission update (recall) and while there I had them address the rattle I've been hearing from the rear subwoofer.

The rear subwoofer would rattle with the volume set at 10.
It was driving me nuts.

Upon inspection the tech found the rear metal of the parcel shelf was cracked where the sub mounts to the shelf.

Not sure if this is due to the bolts being tightened too tight or the metal not being strong enough.

So my car is headed to the body shop on Monday to get fixed.

 

I had my dealer install dynamat all over my rear shelf. All rattles gone, plus the benefit of a better and tighter bass. Just gave a $20 tip and all was good.