Ok, so I replaced the aux belt and tensioner which was noisy and I'm pretty sure its not the alternator.
My question is, what would create this whirring noise, its temperamental, I can start the car up and it'll make this noise other times it won't, doesn't seem to matter if its hot or cold.
I'm wondering if its a potential bearing failure on the shaft that leads to the alternator or a turbo failure. Definitely hear it at idle and on the move.
I have a 1.8 TDCi Mk2 Focus (56 plate - Jan 2007 production)
Just to make sure there is no confusion with the bearing on the alternator; the aux belt drives a shaft which then links on to the alternator which is independent to the alternator.