Thanks. I recently replaced the blower fan and regulator, so I'll check out the aux heater pump. Does that operate without calling for any hot air? The problem occurred with the temp selector in full cold. The humming seems to be coming from under the dash, right above where the temp controller is positioned.

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