I have this sound coming from the engine bay that I can't pinpoint. It sounds like a relay clicking on, then something engaging, then shutting off. It happens once about every 15 seconds. I've heard it might be the a/c clutch, but the a/c isn't on. Any help would be greatly appreciated.

 

It does sound like an A/C compressor cycling. You may not have the A/C control set on A/C but I believe the A/C still comes on if you have the defrost on. I think it helps remove the humidity from inside the cab so the windows don't fog up. Make sure your climate control switch is on off and then listen for any cycling/clicking under the hood.

 

also could be your cooling fan clutch cycleing too, just pop the hood and see if the fan goes on/off with the clicking

 

Dynotec is correct. The compressor cycles even if you aren't running the a/c. If the sound goes away with the air/heater off then that's what you're hearing. Nothing to fix.

 

The a/c compressor cycles at regular intervals like you described. Watch your a/c compressor with the engine running. You'll see the clutch engage and disengage.