For a while I have been having a loud grinding sound, relative to engine RPM which I had concluded was a bad accessory belt bearing. Well sure enough the friction wheel and tensioner failed, belt snapped off and both needed replacing. After replacing the friction wheel and tensioner, the sound remains... The alternator seems quiet at idle, even when using a screw driver as a stethoscope and listening closely. Where else could this noise be coming from?? Thanks!

 

Anything that turns when idling could have a bad bearing. Alternator and AC compressor both become candidates. You have the right idea using a screwdriver (I bought a mechanic's stethoscope for $12) but be careful with fingers and clothing while you are checking for noise.