Has anyone experienced what sounds like a persistent slapping of the rear seatback against the back wall of the cab when running over washboard or potholed roads? (2006 Ram 2500 ST). There is very little clearance between the two surfaces, so whatever is making noise is not readily visible. Repeated searching and manipulation of the rear seat assembly has failed to indicate a reason. Truck is 2 weeks old, and has less than 500 mi.