Loud Clicking Noise When Rear A/C is on

I Have a 2002 Trailblazer and when i turn on the rear A/C there is a loud clicking sound coming from the back of the truck. It is on the passenger side up by the interior light in the third row. If i turn off the Rear A/C or turn the knob and have both the overhead and the feet blow out it does not click.

Anyone have any idea what causes (or solves) this problem? My GF is having the same issue with her 2004 TB EXT. When she first starts up, it makes a loud noise in upper, pass side, rear of cabin. Like up in roof liner, or rear A/C ductwork. It is a rhythmic, whap-whap-whapping sound, about 1x per second. About a block later, it either quits, or doubles in speed, to twice per second, then eventually stops. Next trip, same thing.
Tried messing with the different HVAC controls for front & rear, doesn't seem to affect it.
Have searched the boards, this was the only thread I could find that sounded like same problem.

Jim, sounds like a mode door actuator similar to what can occur in the front temp door actuators.

Agree, rear a/c mode door actuator keeps running and can't find a "park" position so the door keeps flapping. Pretty common on gm's and fords. Replace the actuator and all should be good.