I have just discovered a noise under the hood for the first time. 2001 F150 Supercrew, she has 107,000 (first concern with the truck), she making a noise.. Thought it was a fan noise, however, it's not the fan. Could it be the rocker arm?
Clueless

 

"A noise" isnt very specific. The engine running makes a noise.

What kind of noise is it? metallic, plastic, is it rubbing, does it change with speed or RPM, what part of the engine bay is it coming from...

You gotta be more specific if you want us to be able to help you out.

 

If its a light ticking, it could either be injectors or a lifter. I'd start by checking those out.

 

It originally sounded like a fan running with something hung up in it as you turn the key off. The noise is located on the passenger side between the compressor and the valve cover. Gargling noise maybe fuilds...