Whirring Noise when accelerating

[RatedRVD]

Hello

I have 2008 Yaris 1.0
Its done 10k miles.
I've noticed today that when i accelerate, i can hear a whirring noise coming from the front of the car.
If i put the clutch down, the noise stops, until i take my foot off the clutch again.
If i have the car in neutral and rev the engine i can hear the sounds
I've had a mechanic have a look and he can't see anything wrong.

So any ideas?.

Thanks

EDIT: It sounds more like a whistle blowing,