I had the same sound, As per Turbo gurus, there is still too much presure left in the intake tubes after the Throttlebody closes. The sound you are hearing, if you removed the factory bov, is the turbo being stoped by the air pressure left in the intake. you may need to instal a biger BOV or adjust the one you installed to open quicker. At high pressure, the turbo can explode and throw pieces of turbo to the engine.
$0.02
Ian
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