2001 Sierra 2500HD Whiring at back of engine

[Upanddown]

I have A 2001 Sierra 2500HD that has a inconsistent whirring issue.

 

When the engine is idling or revving down, I get a whirring that comes from the back of the engine bay. If I tap the gas peddle or turn on the A/C it may go away for a few seconds to a few minutes, no pattern. 
 

This is definitely coming from the engine bay. 
 

Any suggestions on what may be the issue?

[davester]

I would suggest getting a mechanics stethoscope to narrow down where the sound is coming from...

[CamGTP]

Pop the oil fill cap off, see if that makes it stop.