Periodic clicking sound from under hood

[oakbough]

After doing some maintenance on my truck and running it with the hood open I notice a distinctive clicking sound originating under the hood. It occurs fairly regularly about every minute or so. I thought at first it was coming from the AC unit but now I cant tell. It could be normal, but as I am learning more from working on the truck I have been paying more attention to sounds and regular operational feel to try and anticipate any oncoming issues. I know this is kind of vague but can anyone explain this? I tried searching for it but all I found was clicking wheel related issues.

Thanks

[timmyboy76]

GEM module? Is your "airbag" light on? Are you hearing 5 sets of 5 beeps? Did it rain recently?

[oakbough]

Airbag light only comes on for a few sec when I put the key into the on position. I don;t hear any beeping. The GEM is located at the fusebox inside the cab right? The sound I am hearing is under the hood and is a loud but quick "crack".

Since the serpentine belt powers the AC unit, is the AC chamber (black cylider on passenger side engine compartment) supposed to be cold all the time or is the GEM supposed to activate and charge the AC only when the switch is activated? The AC unit in the EC is always super cold as though the AC in the cab is turned all the way up.