The Bwahh Stutututu sound is actually compressor surge meaning the BPV/BOV didn't vent quick enough or long enough so some of the pressurized air hit the throttle body, when it slammed shut, and went backwards. Back into the outlet of the turbo which usually isn't good for journal bearing turbo's (Most Subaru OEM turbo's are journal bearing).
A BOV on a Subaru is not a good idea. If you want to here the blow off noise of a BPV, the safest way is to get a well know aftermarket intake and get tuned for it. The car will feel a lot better with a "Stage 1" tune plus an intake anyway. Find a reputable tuner first and ask them what brands of intake they like best. If they like a brand better, that means it is easier to tune for and therefore safer for the car.
If you really want that compressor surge sound, go pickup an old tuner car (240sx, Supra, R32 GT-R/GTS-T, something with a 1jz/2jz motor, SR series motor or RB series motor, etc.), put a big diesel turbo on it, delete the BOV/BPV, tune it and have fun. :thumbup:
-Ty-