You may have read another thread of mine. Car makes a whizzing sound when boost is above 5psi and power is flat from about 4500rpm onwards.

Boost gauge says target boost (10-11psi) is being reached and is stable, which I don't quite understand (how can the power be so flat when the manifold is at target boost?)

No spikes, dips or anything noticabley weird. Motor builds good power until just after the whizzing starts and is flat from there. Seems the manifold is seeing the target boost but it's leaking before it gets into the cylinders.

So far I have replaced all exhaust gaskets from exhaust manifold to dump, replaced pipe from turbo to intercooler, removed the bypass valve (aftermarket plumbed blow-off) and had the intercooler leak tested.

By my reckoning this leaves only 2 possibilities:
1. turbo is dying
2. an inlet manifold/throttle body gasket is leaking

How might I work out what it is wrong before I tear off the turbo and/or inlet manifold?

And if the turbo is dying, what exactly would be going wrong?

This is pissing me off!

Here's a video of the sound: http://www.youtube.com/watch?v=cNsq3GOrdfY