E-Box Fan.

Common issue.

Seach here, or mbworld.org or benzworld.org.

15 min DIY job.

Remove fan, clean and replace. Sound will probably stop. The replacement little squirrel blower is like $150 (ouch!). I decided not to replace. Clean and reinstall.

My noise stopped shortly after cleaning.

Dickie
00 ML 320
97 328i
87 325is