Strange Whine!?

[Ruan]

Turns out its easier to get at the sprung tensioner from underneath.

Found out it's the alternator making the noise, it appears to have some sort of one way clutch so it only spins one way, allowing it to slow down much slower than the engine itself. Now not sure if its a fault or if its just normal alternator noise that would usually be covered by engine noise!

Yeah, was just about to suggest that..

Quite common now, it's to stop the belt flapping - as it's the magnets going past the windings that effectively make it shake since it's a 6 pole - you get 6 shakes per rotation, so essentially it decouples it and allows free play.