I've tried searching for a few causes to this symptom.

Randomly as I'm driving down the road (even with the A/C off) I hear a single, random click every 20 seconds. It seems to wear off as the truck gets warmer, but I'm having trouble narrowing it down.

I am kind of thinking it has something to do with the big clunky cooling fan clutch. I almost never drive with my A/C on, but I know on my 95 Cougar the compressor would sometimes just kick on even when in defrost mode 100% heat)

Anyone have anything they can offer me?

 

My 06 F150 screw has a similar random click noise. I'm so lost as to what it is, because the truck won't even be on and it will click. Please tell me it's nothing serious. I like to fix the problem, if there is one, and move on.

 

The 5.4's do use an electric fan clutch (not sure if the 4.6's do, but the 4.2 V6's do not). You maybe hearing the fan clutch engauging/dis-enguaging.

If you have the defrost running, it's the AC clutch, as Ford and most other manufacturers do this to keep the AC seals from not getting oil and drying out during the cold season. It's normal if this is what your experiencing.

 

My rangers alternator used to click on and off as needed and was similar to this. About every 20 seconds but always scared the crap outta me if I was standing next to it with the hood up. Probably not a big deal. Probably just the fan like Bucko said.