I had the same problem with my '80 300D. At first the window would slip down a little while driving and do what you described when raised.
I pulled the door panel and found the teeth at the end of the window regulator worn. I checked my spare Benz in the woods and found that was also the case with that one. Strangely, this is the only reuglator that showed this type of wear.
I priced the part and it was over $100 so to the bone yard I go. It took a bit of searching to find a left rear regulator that was not worn in the same manner. I did find one with a little wear for $12, installed it and now no more "dropping" window. Since the wear is on this one also I still get the strange noise you describe. I'm just careful to release the button after the firts bump.
I hope this helps.