Whistle

[lynns hubby]

My daughters 02 1.9 Turbo diesel has just developed a whistle. It is throttle dependant. When I say that, I mean I gets louder as the revs increase. It sounds to me like an Air leak somewhere.
It does it if you rev it stationary on the drive and it does it as you drive. I can hear it as she pulls away from our house. I have checked all round the turbo and associated pipework for leaks induction and output side. I cannot find anything obvious. It does not effect power which if it was on the output side of the turbo I would have expected as it would not be delivering the correct amount/pressure of air.
It does not lack power in any way however she tells me it is using a little bit more fuel.

Any ideas would be appreciated as usual.