This is a common problem among ALL W124's. Unfortunately, there is no good fix for it. I have been told it is necessary to remove the door panel and then tighten down a screw on the inside.(and add locktite IMO) Probably more trouble than it is worth. You can always wedge a bit of foam/felt in the edge to quell the noise short term. Unless Benzmac or MB DOC knows another way to access that area...that is all the help I can offer.
Hope this helps...Lee