The update: The blower motor's bearings were making the noise, so I decided to change the motor. It was easy, about 20 minutes job, but what a surprise, the new blower motor is making the same high pitch noise at the low speeds! What is wrong with Mercedes? I am going to try to exchange the motor and see if this will solve the problem. May be I got defective part? The funny thing that according to Mercedes service records the previous owner had the same problem with blower motor 13 months ago.
Did anybody experience same problem?