That BOV appears to have an outlet which is intended for recirculation... It's not built with making a lot of sound in mind. Fill you mouth with air like you're going to whistle or blow up a balloon and then suddenly, open your mouth all the way... it's a lot more effective at getting rid of the pressure but is a lot quieter than whistling until the air is gone.
It also appears that the BOV has an adjustable cracking pressure; You adjust the nut to determine at what level of vacuum it opens the valve. This is used to keep the valve open under low boost conditions where surge isn't as big of a concern in order to improve response between shifts. Basically, the higher it's set for the higher the amount of boost needs to be before the shift for it to open. If it opens too early the valve may remain open while at idle or really ruin your performance during casual driving... both are bad but the valve remaining open is VERY bad if there's no filter on it. Refer to the manual for information on adjusting the cracking pressure or just experiment with it.
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