My experience with door rattles in a 107 SL has been the same culprit on two different vehicles. The guideblocks in the bottom corners of the glass that both isolate from and guide the glass up and down the steel channels come loose and fall off. I found one in the bottom of the door, on the other car it had long since vanished (and I had to fabricate one out of nylon because they cant be purchased as a seperate part). They are glued to the glass with a urethane type rubber and I guess they fail with age. This leads to some pretty significant rattling as the glass drifts back and forth and hits the metal channel. I would guess that you could be unlucky enough to shatter the door glass if you slammed the door if this problem isnt rectified.

Remove the door trim and look in the bottom of the door ... if you find a die cast metal block in there thats the problem. To reattach it is quite a process as the door glass needs to come out. But then again if you have removed the door trim you have already done about 40% of the work so you may as well finish the job.

__________________
'71 350SL auto
'73 350SL manual
'98 528i (yeah I know but at least its German)

'71 250CE (sold)