Loud knocking in dash when vehicle is started and no air from front vents. on 2003 GMC Envoy

When you start the vehicle there is a loud knocking that goes on for about 1 minute. There is no air coming from the front vents. What is the thing that shifts the vented air around? I wonder if it has somehow become jammed? Air works from floor and windshield vents.

Asked by for the 2003 GMC Envoy
your blend door sctuator has gone bad, that regulates air flow from heater to defrost and a/c. approx cost is 400.00 to replace it.unless its the one that is in the middl of the dash and that one is approx 800.00
I had the same problem with my 2003 GMC Envoy..... knocking (clapping) sound under the dashboard once you turn on the ignition and air is only blown in the dashboard and floor area... what I did today - (July 14, 2014) perhaps by stroke of luck - was only to change the module that controls the a/c system.... NO MORE KNOCKING/CLAPPING SOUND AND AIR IS BLOW IN EVERY DIRECTION THAT YOU SET.
2 more answers , 3 more comments
Loss of use in the HVAC directional vents will usually mean a bad mode actuator that needs replaced.
the blend door broke off...and is flapping inside the car.
I wonder if there is a way to reach in and remove the broken part without pulling the dash. Not really concerned about the vent but the knocking is pretty annoying.
There is a way to eliminate that annoying knock, try to access where the door or actuator is and use self-tapping screws to wedge whatever is making noise.
this swill stop the noise nut not fix the problem
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