Rear compressor clicking?

[kickapebble]

I noticed a "tick" sound every secong emitting from where the rear a/c compressor is. It is only ticking when the overhead exhaust is selected, when on floor, it seems fine. While on overhead, it seems that the air is coming out just fine... any ideas? 2004 XLT.

 

[ELVATO]

http://expeditionforum.com/showthread.php?t=2324

 

[kickapebble]

Thanks, great info. Seems to be a common occurance.