if it was the flap wouldnt i have increased boost than? mine stays on 8 psi all the time. got no performance issues really, just that awful sound.
When the car is idling you can slightly hear something blowing, if you rev it a bit it gets abit louder but on flatout riding its just so loud really similar to what some people call subaru noise?
Going to work on monday ( free day for me ) to get it on the bridge and look from underneath maybe ill find something..

thanks