Whistling sound, Possibly from Clutch or belt

[Nate]

I've just come back from dropping my mate of home he was on the back of my scoot it's a Gilera DNA 125.

Now when I twist my throttle to activate the clutch or pull away I hear a faint whistling noise don't know how best to explain it I think probably if you blow into the top of an open bottle that sort of whistling noise.

The bike rides as it should and theres nothing to suggest anythings wrong with it but I've never heard that noise before anyone have any ideas as to what it is?

P.S: It only does it for about 5 seconds during acceleration.