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Rayman, does it sound like a siren or a sweet whistle? It's not just the turbo winding up under load when you accelerate? Does it make the same sound all the time, ie at idle, under load, cruising?...
If it’s on spool up its most probably bearings in the turbo.
If it sounds like an old WW2 air raid siren a squalling ringing metal sound I would say bearings
If it sounds like a hissing nose then...