What else have you done to the car recently? I'd be inclined to think it's a vacuum leak somewhere at one of your intercooler piping joins or something similar.
I had a shitty high pitched whining noise coming from where my bov meets the crossover pipe, turns out the gasket wasn't sealing it properly so when there was 7+psi of boosted air going past it, it would leak out and make a sound like air escaping from a tiny hole in a balloon.