Wierd problem with Air condition on C280 1999
 

I can hear a wierd, high pitch and very loud annoying sound everytime I turn on the air condition on automatic mode. I has something to do with valve management. when the arrow on display points upward, there is no noise. in any other positions I can hear the noise. I has nothing to do with the fan speed. min or max you can hear it the same. any Ideas?

Belt?