The rear ac on my 03 xlt expedition makes a loud clicking noise about every 2 seconds and stays at the 2 second rate at all fan speeds. The ac for the truck is manual. The clicking starts whenever either front or rear control turns the fan on and at all speeds. It starts softly and progresses to loud and annoying. The front works just fine. I took the plastic package tray off of the top of the rear unit and there is just a large plastic shroud with the clicking sound coming from somewhere inside. I would appreciate any suggestion.
cc

 

Sounds like a blend door(?) actuator going bad and the gear drive skipping. This is a total guess, though. I have never dug into one of those.

 

That sounds right. It started happening every so often and then would go away. It has now come back with a vengence. The period on the clicking cycle time is about one second, I timed it. I think I'll let the dealer handle this one under warranty.
Thanks again

 

The dealer comfirmed your diagnosis and replaced the gear.