My guess is it is your aspirator fan motor. They start to get noisy when they go out.
It is located next to the glove box and behind the passenger side air vent.
It's purpose is to blow air up through the "A" pillar, through your head liner, and out the vent next to your sunroof. The temperature sensor is up there for your HVAC system to tell your system what the temperature inside the car is to dictate whether to blow hot air or cold air.
2001 E430, Bourdeaux Red, Oyster interior.
1973 280SE 4.5, 170,000 miles. 568 Signal Red, Black MB Tex. "The Red Baron".