Do all of your heater controls work properly? For example: If you set the AC to blow all out the upper vents, does it blow out the floor and/or defroster outlets? What kind of noise does it make when you shut the car down? Does it stop immediately, or does it continue for a second or so?

I'm going to agree with Scimmia and say it's probably a vacuum leak at or near the vac port of your heater controls.