W126 Noisy rear seat
 

My 89 560SEL with 71k has what appears to be a noisy rear seat. The passenger top side of the seat could be moved backwards and forwards as it appeared to not be seated correctly. I removed the seat and reinstalled it and it no longer moves, however the noisy seat still remains. By noisy I mean while driving down the street when running over bumps and dips in the road the seat makes a general clatter. It doesn't happen with passengers in the rear. While I had the seat apart I checked all the brackets etc and everything underneath is secure. Does anyone have any idea on what to check next?