Electric motors bearings wear out.
I did not see how old your car is. But I have had fan motors start making noise after only 4 years. I would guess your bearings are worn out and the motor is vibrating (That can sound pretty violent.) The other possiblity is that something worked itself into your ductwork and is hitting the squirel cage, but most MB's have a filter on the air intake. I would call a MB dealer and see if they sell bearings for that motor.