My buddies 03 f150 with the 5.4 makes a ticking/clicking sound whenever he runs the heat or air through the defrost or vents. It's not very loud but you can definitely hear it and it comes from the passenger side under the hood. Any ideas??

 

It's the ac compressor. I found it comes on for every vent option but panel.

 

Heater core or air in the lines would be my guess.

 

Best test would be to turn on the vent options that cause the noise and pop the hood. If its the compressor you can see it spinning on the bottom of the engine block on the passenger side. And it shouldnt be constant. Mine turns on and off and you can seeit engaging and disengaging. It will have two or three bolts coming out of the front.

 

The ticking happens on panel also.