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

I should say that this first happened immediately after I made a hard launch. The first time I'd ever really given it a bootfull off the mark and took it to redline.

I think maybe you do have a boost leak. The leak isn't bad enough to stop you from hitting full boost but because air is leaking out but has still been measured by the AFM, the ECU adds more fuel causing you to run really rich and the engine feel flat when on power.

Are you still using OEM boost control? Still have the original TMIC with plastic pipes going from the turbo to the TMIC?
That pipe gets really hard from age and heat. They have a tendency to crack.

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RS Liberty - Because WRX's are only good for parts...

Yes, OEM boost control and TMIC. Just replaced turbo-to-TMIC-pipe with shiny new alloy one. All other hoses are good, the only one I haven't yet replaced is the stubby one between the TMIC and throttle body but I'm reasonably sure it's not leaking there.

Check the gaskets for the inlet manifold to the heads.

The only other thing I can think of is a damaged turbo. If the compressor wheel is damaged, it can make a sound like that.

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RS Liberty - Because WRX's are only good for parts...

I've owned this car for a 9 months, it's in pristine nick with only 100,000 on it. Belonged to a middle-aged doctor. The only non OEM part on it is the cheap aftermarket BOV and the new alloy TMIC inlet pipe I just fitted.

Let's say this was the first time it had ever been run super-hard. Reckon the cheapo BOV might have failed to open as I shifted from 1st to 2nd and damaged the compressor?

what turbo is on it, that sounds like my race car which hit's boost then the waste gate and boost control kick in, however that car boost hard and normal.
but the other thing I can mention is my road car that does the same thing hit's boost hard in 1st and second then in third it feels like the nachor has been thrown out, so maybe when it's fully on boost the car is supplying the power at a constant rate and you don't get the massive accelration feeling.

Las t option check the aftermarkket BOV and make sure it's on good and tight and the air isn't leaking out around that gasket when boost is prolonged.

oh forgot one, hows the boost gauge plumbed in could be leaking fromt at too in the cabin

Remove the BOV completely and block and openings left behind from the removal.

That will either rule it out or point it out as the cause.

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RS Liberty - Because WRX's are only good for parts...

+1. funny stuff happens also when BOVs are plumbed back to front also. def try this.

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1997 green GC8 with the usual mild mods.