I'm having a sound happen whenever my cherokee (94 4.0 H.O) runs now that sounds similar to the sound a power steering pump makes when it gets old or goes bad. (It's not the pump, haha.) Kind of a low humming noise. It's constant when she's running, performance hasn't been affected, and I have no idea what it would be. Anyone have an idea???

 

Take the belt off and free spin everything with your hand, make sure everything moves smooth.

 

Figured it out! Got to thinking simpler and remembered my cheap A/C compressor bypass pulley isn't the highest of quality and I had to grease it myself, shot it with some PB-50 (PB blaster wd-40 equivalent haha) and it shut up. Was the plastic and bearing moving against each other and making that odd harmonic. Phew!