i have a similar problem. it's a high pitch constant ringing noise. sometimes the sound would happen even with the climate control on. took it to a indie shop for mercedes only and i was told there's a leak somewhere. don't remember what exactly it was, but i do remember it would run about $800 or more to have it fixed. so i did't bother.
ur problem might not be the same as mine. so check with your local shop. some of the guys here might have a similar problem as well.
by the way, mine is a 99' c280 sport, with lcd climate control.