My mother's E320 had this problem, and I found that 90% of the noise in her situation comes from the plastic covers at the base joint where the back and bottom sections meet. To solve this I had some extra black rubber-backed outdoor carpet I used to cover some sub boxes, I then cut them to fit the outer shape of the plastic and inserted these cutouts inbetween the plastic and the seat with the rubber facing the leather for a little grip and the carpet isolating the plastic. Her seats are black, so this worked well in this case and solved the problem lickiddy split (20 min).