Joe,

It has been a while but I believe the wiper arm attaches to a splined stub shaft, and the connection is covered by the black trim part on the wiper arm. The trim piece pivots up to give you access to the splined connection. Sounds like yours has possibly unseated. I think the fix is to take it off altogether, clean up the splines, and then refit and retighten it.

Like I said it has been a while, but I am fairly certain yours is the same as my old 240D. Good luck, Jim

__________________
Own:
1986 Euro 190E 2.3-16 (291,000 miles),
1998 E300D TurboDiesel, 231,000 miles -purchased with 45,000,
1988 300E 5-speed 252,000 miles,
1983 240D 4-speed, purchased w/136,000, now with 222,000 miles.
2009 ML320CDI Bluetec, 89,000 miles

Owned:
1971 220D (250,000 miles plus, sold to father-in-law),
1975 240D (245,000 miles - died of body rot),
1991 350SD (176,560 miles, weakest Benz I have owned),
1999 C230 Sport (45,400 miles),
1982 240D (321,000 miles, put to sleep)