@Tonks could possible be a boost leak close to the turbo as @MudRunnerTD stated.
The main point of my post was for you to check and make sure that the exhaust manifold is not leaking at the head interface and also check at turbo outlet flange that mates up to the exhaust manifold. Also check turbo to dump pipe flange gasket.
Take the induction pipe of inlet side of turbo and place your finger on the shaft/spindle end at compressor wheel and try and move it sideways and axially, you should be able to guess how much play there is(there will be some). They can handle some play, look for a dulling of the turbo housing bore adjacent to the compressor wheel. The shaft does centralise (run true) as it spins up.
I stopped the noise by reducing boost at idle and automatically thought the turbo was defective but ended up being a warped exhaust manifold.
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