Many of these cars have a problem with rattling rear shocks, and there is a TSB on it. Mine had the shock rattle, in fact still does. Dealer has put new shocks on twice, and the difference is wonderful. Unfortunately, after a few thousand miles it comes back.
The rattling in the rear shocks sounds just like a door rattle; I guess the sound travels. In fact, I spent time taping up my latches, etc, etc, because I was convinced the rattle came from the door.