How about you come over here and visit me. I'll run an example for you.
Here's how I'll do it.
I'll start my car, put her in neutral and leave the handbrake on. I'll pop my bonnet, rev the thing to say 4000-5000rpms. Air filter makes a flutter noise. (Car didn't make boost, bov doesn't open)
Now, we do it again. This time I rev to 7500rpms. Car will build boost (<5psi), BOV opens. Two noises can be heard. One you already have heard, the air filter. The other is the BOV.
Take note of the noises.
Go for a drive, take note of the noises upon releasing the throttle, sounds exactly like the BOV.
Still don't believe me, then that's your problem.