I am not there but I suspect that the "flap" is the vent door that diverts air from outside to inside.
There flaps are driven by a motor attached to each area, defrost, heat, air etc.
In the area of the flap noise unplug and remove the small, flat black motor and move the arm by hand. If you can replicate the noise then you have isolated the problem.
Regardless you will have to remove the dash and then the entire A/C-heater assembly uless you can find some way to block the flap from turning.
It is a 12-15 hour job because the A/C has to be discharged, steering column out, dash out and then you can get to the assembly.
Hopefully you can find which flap is loose and block the movement. One of GM's better ideas.
Monday, May 22nd, 2006 AT 8:07 AM