I think I know what you are talking about. The passenger seat has been doing it for a few days a little while ago. This happens when I come to a stop. With someone sitting in it, of course (usually my wife). It doesn't happen constantly, and hard to reproduce, even when trying to move and shake the seat violently. I was about to get mad and drive to the dealer but then it stopped. Just like tooth pain stops when you are about to go to the dentist.
I drove yesterday with my wife, and didnt hear this.