I have a 1985 300CD that when bought (last summer) had very unpleasant whistling from passenger side window at speed. When I looked closely at window weatherstripping, I noticed that there were small gaps (1/16 to 1/8 inch) in it where sections were joined. I think from just being old and shrinking.
I used some silicone and carefully filled in the gaps. No big deal if you are careful. Can get the stuff in small tubes with narrow tips. I used clear buy if you can find black, would be better. Silicone is flexable and completely waterproof.
Virtually all whistling now gone! Small openings make big noises.
'85 300CD @ 138k miles