Welcome back!

Well you are right about the motors not being repaired however I have heard rumors about people changing the brushes. Unfortunately from what you explain the brushes are not the problem.

Usually when the motor starts to make noise like that its time for a new one. Having had many older Mercedes in distress I have experienced this problem in the past before also. What happens next is the motor heats up and starts to melt the plastic casing and boy does that ever emit a nasty smell thru the vents. Best to replace it now before this happens. Also its important to remember that a blower motor with bad bearings puts excessive load on other components such as the blower speed control, blower fuse and even the climate control panel.

I have attached some photos for reference. I think that for the most part yours will look the same. The photos I am attaching are from 1986-1991 models as usual however if anything looks different feel free to post some more of them good photos.

The blower motor is located on the passengers side under the dash, you will have to remove the panel just under the dash and you will gain access to the 4 Philips head screws to remove the cover. Let me know if anything looks different on your end. The blower motor speed control is the part that looks like a triangle bed of nails.