whistling ??????

Okay...i have a 2001 pontiac grand am se with a 4 cylinder motor. The problem is when ever you turn the knob to air or vent you hear a whistling....it does it even without the blower being on....I was wondering if anyone might be able to tell me what to look at that might be causing this. Thanks in Advance.

yea thats an easy one it has a vacume leak behind the heater control panal take it all aprt down to the heater control turn it on find the leak and replace the hose engine must be runiing to find the leak