I experienced the same sound in the low position. I lived with it for about six months and then I got tired of my friends asking me what the squeaking was. It became an embarrassment so I finally replaced the blower motor. It solved the problem. Sounds like yours ceased up completely. You will be amazed that the blower motor will come out from underneath that small place under your windshield.

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