I installed a 12" subwoofer along with the already stock one and I hear a rattling in the back of the car. After some trial and error I just placed foam in the back edge of the deck near the window so when the bass hits it wouldn't hit the window and make a rattling. Also I put foam on the border of the inside breaklight. No more rattling

 

I had the same problem. I just ended up getting rid of the factory sub and adding a bazooka in the trunk. The factory sub is kind of weak sauce anyways. I loved my 3G TL but had to hunt down about a dozen rattles over the three years I've owned it. I've literally had 0 rattles in the 4G TL so far.