the sound is present all the time, but louder when the A/C kicks in.
The alternator is a new unit (3-4 months old) though that does not rule it out.
unfortunately a serpentine belt runs several items (including alt and a/c), so taking it off will not help differentiate between the alt and compressor, but I could maybe spin the alt by hand and see if it "feels right"

The belt is the original belt, but it looks fine and seems to have the proper tension. The noise is mote metallic that the "belt type" noises I am familiar with.