Alternator replacement ?

[SeaGrey]

I've recently acquired a C-Max with 1.8 Duratec HE (66k on clock). It runs very smoothly, but at idle it sounds like a pulley bearing is knackered (typical noise coming from aux belt area).

My C-Tek charger tell me that the charge voltage has an issue, and I've measured this at 13.8V with the engine running.

My question is this: Is it possible that a bearing fault on one of the pulleys (or the component turned by the pully) is causing drag on the aux belt, which in turn causes the alternator to turn too slowly, hence the low charge voltage ? I actually doubt this, as I still only get 13.8V even at high engine revs.

Obviously, there's a possibility that the alternator itself is on its way out, but is there any way to isolate it as a cause of the problem ? I'm reluctant to replace it through a process of elimination, only to find there's something else causing the noise, e.g. water pump. But perhaps the 13.8V output tells me it needs replacing anyway ?