Its been wet here a lot in Cali and I have noticed a rough grinding sound coming from the base of my windshield wiper on my 92 w124. I keep the piston at the base of the arm well lubed so I am sure that is not making the noise. I cant figure out how to get inside the base of the arm to see if I can lube it all up. it has not been done in 18 years I'm sure...

The other post's I find talk about 2 tabs at position 3 and 5 I believe.
I cant seem to find them, or the 92 is a different design. some are a sealed system I saw somewhere and they said they cant be opened, I would not think a 92 would be...

Though I don't believe lubing it up would get it to w210 speeds I cant afford to let the motor work so hard to move the arm and wear it out... and I hear the grinding over my music... and no turning up my music is not a fix LOL! although it IS a fix for a few other problems!

 

Not sure if you figured this out or not but I just went through this with my 92 400E.

if you don't see tabs on the wiper arm cover, you've got the version where the cover does not come off. The versions with tabs are plastic---yours is metal---and were used on later years.

I ended up having to take this to my mechanic who replaced it with a used system (that had a plastic cover). Whole assembly was $500 plus labor.