sort of sounds like there is no blow off valve and the pressure in the intake is forcing charge air back through the turbo.
My dad has a ford 7.3 L Powerstroke diesel and it has no method of purging the intake of charge air when you lift your foot off of the accelerator pedal abruptly upon high boost, so the pressure inside the intake becomes higher than that of the pressure in the exhaust. this tries to spin the turbo backwards and makes the whistling noise when the air flows over the fins backwards inside the turbo as it is still spinning at +100,000 rpm. it kinds sounds cool on my dad's truck... but some say it could really damage the turbo.
I think this is what's happening just by comparing it to my limited experience with turbo's so you could call:BS if you want lol
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