Today, I noticed a humming noise that seems to be coming from somewhere under my dash board. It sounds like maybe a motor or servo of some kind. When I turn the cockpit blower fan completely off, it goes away. Then, when I turn it back on (either in just fan or a/c mode) it takes a few minutes for it to reappear.

Any ideas?

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1993 400E, 256,000 miles (totaled)
1994 E420, 200,000+ miles
1995 E420, 201,000 miles