I have the same rattle. It is still there but these are some of the things I did while investigating:
a) when i took off the door skin and plastic, I found 2 screws laying at the bottom of the door cavity. with a shop light and operating the window up & down, I found the two places they came from. tightened them up and also tightened every other screw I could get to. note that some are accessible with the window up, and some with the window down.
b) I fully greased the channels where the window rides and all the grearing spots on the window crank
c) I noticed the door lock wire has play in it where it can clank against the door frame. I should have wrapped foam around it, I'll go back and do it this winter.
d) the door itself is not snug against the body when closed. there is about 3/8" of movement if you lean against the door. the drivers door does not have this looseness. I don't know how to remedy this because I don't see any adjustment possible on the door latch. someone else recommended stuffing a rag and then closing the door. then take a ride and see if the door rattles. if the rag stops it then the door is loose.

good luck
Jack
85 380sl