I agree with the others. Probably some debris is blocking an air blend door from closing completely and the air leaking past that door is creating the whistling noise.
Perhaps some of the foam rubber door seals has broken off and causing the leak.
Another possibilities is one of the vent pipes has a small leak where it connects to the duct.
The third possibility is leaking vacuum line controling one of the air blend doors.
To help locate the problem, turn the blower on high, and start changing the vent locations dial so that you can listen to the floor vent, the dash vent, heater vent, recirculate vent, etc. When the whistelling stops, you have found the offending door and can then better assess how to fix it.
...Rich