[LuxoM3]

Wizper_F32:

I agree with you, but thinking it's NOT your turbo.

I was shaving this morning and thought about your post some more. I had an old turbo kit from GReddy for my Miata years ago and that kit was like a stage 1 without a blow-off valve. So whenever I got off boost, I'd get this whirring sound because the boost pressure had no where to go. Once I got the upgraded BOV, sound went away.

"I recently installed a plumb back turbosmart BOV"

Given this is literally your own change, I highly suggest pulling the new Turbo Smart valve off and putting your stock one back on first before thinking it's the turbo. It's a quick, cheap and easy first diagnostic step.

Good luck man!