Seat back rattle

My 2011 2.7 AC 4x4 has an annoying passenger seat back rattle. When empty (no passenger), it shakes back and forth quickly on the bumpy back roads I live on, and it makes quite a racket when it does this. I just want to know if other folks have experienced the same thing. I do not want to bring it back to the dealership if this is normal for this truck.

Thanks!