Update - looking at another site, this seems to be the problem:
"Window operates ok except for one hell of a clunking/grinding/banging noise when it gets to the top. REALLY common on W123 cars, especially rears. It's probably because you ignored the fact that the bolts holding the riser to the door were loose. I've even seen them crack the inner door shell. The torque of the motor twisted the beejezus outta' the cast bracket part of the riser assembly....and it cracked. (I've got used ones that aren't....)"
Go this from
http://www.msdra.com/mercedes_power_windows.htm
When I am careful closing the window, i.e. I am letting go of the button just as it reaches the topmost position, everything is fine. Has anyone dealt with this one on the W126?
Anyhow, it looks like I will have to go inside the door this weekend and see what is going on...