There is a clicking sound coming from the alternator area when in idle. When I put my foot on the gas the sound increases along with acceleration. I heard this once before when driving but then it went away the last 3 or 4 times I drove. Now its back. Any ideas? I have a couple but would appreciate your feedback. Thank you.

 

Does your truck have SAI (Secondary Air Injection)? Could be the sec. air valve on the passenger side head.