You were right... again :)
3 rubber buffers were broken in two parts, the 4th was the only still in one piece.
3 drops of loctite made the job, I feel more secure now.
While the set was opened, I took videos of the moving parts at the 3 speeds. The belt on the right side moves strangely. The set is more noisy when "closed".
Videos are online on this playlist, if you can't reach them just tell me:
Location: Brittany, France
My B&O Icons: