This reminds me of a car I had in the shop recently, with an intermittently squeaky serpentine belt and a slight bit of a growl coming from under the hood.
The air conditioning compressor, alternator, water pump and 2 idler pulleys as well as a pulley on the crankshaft share this belt.
I noticed the squeal mostly right after starting the car on cold dry mornings, when the heater fan was placed on high and the electrical resistance defrosters were turned on.
My mechanic tested the alternator for output (with whatever measuring tool he uses for that) and the output was good, and he tightened the serpentine belt to spec (with whatever measuring tool he uses for that).
The belt squeal went away, but the slight growl was still there. He had placed the stethoscope he uses in such cases (a measuring tool) on all pulleys on that belt, and said it was inconclusive as any bad bearing could transmit it's sound to other bearings sharing the belt. He said however, that when he simply listened with his ears, he thought it "sounded" like the alternator bearing was probably the culprit, so I kept my ear on it too.
2 weeks later the alternator started groaning miserably, and then failed completely. We replaced it and all of the abnormal sounds I'd heard were gone, so I trust we've now solved the problem.
However, the tests he'd run with the measuring tools available today did not pinpoint a problem. His trained ears, on the other hand, had predicted what was wrong.