99E300 - Seat Belt Rattle
 

I'm interested in tracking down a rattle in my passenger seat belt retractor. The noise sounds like a mechanical relay repeatedly firing - best picture I can paint to describe it is imagine shaking a ball bearing rapidly from side to side in a matchbox. Anyway, it's got to have something to do with the electrical monitoring of the seat belt mechanism because the sound ONLY occurs when there is a passenger in the seat, and ONLY when the belt is fastened. My understanding is that there are sensors monitoring both of these conditions. When I sit belted in the seat with my wife driving (good idea by the way...), I hear the noise. If I unlatch the belt but continue to hold it tight around me, the simple act of unlatching the belt makes the noise stop immediately. If I drive around with the belt latched and the seat empty - no noise. The noise seems more related to inertia (another clue that it's related to the retractor locking somehow) rather than road bumps per se. The noise will still occur on a perfectly flat smooth road. Stealership tells me I would need to replace the entire belt mechanism to stop the noise. Go figure. Anybody with any mechanical know-how about how the interior mechanism actually works got any suggestions on how to troubleshoot? Thanks!

Hopeful solution
 

Wanted to update my original post to share my experience. Finally isolated the annoying rattle in the B pillar as being caused by the belt lock rapidly engaging/disengaging because of a faulty seat sensor. Took my mechanic for a drive yesterday and he figured it out. He was rocking back and forth with the seat buckled, whether we were moving or not, and the lock relay was fluttering like crazy. So he's going to pull the passenger seat today and replace the sensor. Unfortuntately time consuming procedure so labor cost will be up there but my car will finally be silent. Just wanted to share....!