I'm not that knowledgeable about bovs, but I believe the stock one is a by pass valve, not a bov.
atmosphere means you're venting the air to the atmosphere (or outside) instead of recirculating it.
recirculating means the air goes back to the turbo when you let off the gas.
aftermarket bov's make noise because the air is being vented to the atmosphere. It's kinda like like whistling.
the most used bov's for the wrx are the Hks SSQV Bov, Forge Bov, GFB Bov, TurboXS RFL Bov, and Worx Bov.