What is this sound?

 

It sounds like a bad pulley bearing to me. You can use a stethescope, a piece of garden hose or a long screwdriver and touch different parts and narrow it down. Obviously be careful around rotating parts. It could be a tensioning pulley for the serpentine belt. Be careful. Put your ear to end of hose etc to listen.

Chris

 

Take the belt off and run the engine for a short bit and see if the noise is gone, if the noise is gone then check all your pulley/bearings for roughness or play