your bov is set too hard and not letting all the air out, and like what someone said (forgot who) its almost like compressor surge... the blades of the turbo cutting up the air that's not all coming out. set it a little lower... i know this because when i had the turbo kit put on my 3000gt, my hks ss bov was making a "fluttering" sound that everyone thought sounded much like a fart... anyway, i loosened the setting and it got it to whistle... same thing with my friend in a subarau legacy that just put a turbo kit on... he has the type s greddy and he gets the fluttering sound if the bov's set too hard... hope this helps....