| It is a sampling fan that needs to be replaced to eliminate the noise. There are two sampling fans, one where you describe and another overhead.
I just had my overhead unit replaced. FWIW, on the overhead unit, it used to be possible to replace just that fan, now the fan is integrated into the overhead unit and the entire overhead unit was replaced ($300 part from MB). It was done under warranty in my car so no $$ out of my pocket. I do not know whether the lower sampling fan can be replaced without replacing an entire circuit board. | 
