Mine started to do the same thing after a 70mm tb install, never found a leak. My guess is the screws in the tb flap were hanging out a bit and could be causing this. I plan on grinding mine down to see if it fixes it but i'll do that when i swap the intake....and maybe heads...hopefully some time this winter.
As soon as you get in there, you'll see the sharp edges. Get rid of them and you got rid of the whistle. No downside to keeping it other than the annoyance