Help - Strange Noise, Think It Might Be Alternator

Just got back from short journey (literally 15 minute drive back from around 10 miles away) - car was fine on the way across this morning, but just before I pulled back into my driveway, a strange noise has started coming from the engine bay.

It kind of sounds like a metallic whistle - like bearings or something similar. Doesnt increase or decrease with revs, have put all the electrics on and off, and there is no difference to the noise whatsoever.

Car is running ok, although I have been felt that its not been starting quite as quick as normally, couple of more extra spins but nothing major.

Can hear the noise in the cabin when idling but its not loud enough to be heard over acceleration - but with head under bonnet it is fairly loud.

Typically there are no garages open near me as its gone 1pm on a saturday and they have all gone home so I wont be able to get the car into Fords till Monday.

Any ideas/experience of this previously?
Thanks