My '01 ML320 with 166k miles has started to make a whistling sound, starting at about 15 mph. The sound is very consistant and increases in pitch until about 35 mph, when it becomes too high in pitch to hear. It appears to be related to drive train and not motor RPM as the whistling sound is not affected at all when I shift into neutral and rev the motor while driving--the sound continues as it would if still in drive. Does this sound like a transfer case issue? Maybe a vacuum leak?

 

I have the same problem did you ever figure out what it was?