On my car, if you have a Boomba BPV adapter and you block the air holes, the bypass air doesn't go back to the filter like stock.
I had the welders at work fill in the three slots on my Boomba BPV adapter with pieces of aluminum key stock, but left a tiny gap at each slot. This caused big time turbo flutter between shifts. Sounds cool, but I'm sure the turbo is pissed. So then I drilled two 1/8" holes at each slot. It has less of a flutter between shifts, but I can hear the adapter more.
I'll try three more 1/8" holes, but If it doesn't get rid of the flutter while having a quieter blow-off sound, I'll just go back to stock.
UPDATE: I drilled three of the holes to 13/64" (had it in the drawer) and the flutter is gone. The BPV sound is about halfway between stock and a Boomba adapter. Perfect for now.