When you turn on the fan for your heat or ac the upper vents (dash board)do not on 2002 Chevrolet Astro

blow out.The box under the dash by the passanger gets loud with the sound of forced air.

Asked by for the 2002 Chevrolet Astro
You probably have a broken vacuum line to the control head. Vacuum is used to control the vent/defrost damper, and if the line is off it won't move. This could be under the hood, or the connector could be broken on the switch itself.