I had the same problem checked by a very reliable and knowledgeable mechanic. His explanation is that whistling that you hear comes from the motor ends (axes on both ends) spinning out of angle because the brushes (if I remember correctly) are worn out and, eventually, the motor will stop.
I am still using the same motor and the noise is on/off.
My car is a 300e -'86, 180k.
The choice is simple: to wait until it stops or do it now.