Stock S4 TII (with S5 turbo running ~4psi) when I let off the throttle to shift at a high RPM (6-7k) it will make some sort of noise I cant describe. Its as if the BOV cant handle the amount of air it has to bypass so im afraid that the air is going back through the turbo (I do not however know what this sounds like). Whenever I make boost at a lower rpm and let off it sounds like air is being bypassed like it should. I was under the impression the stock BOV was supposed to be 'quiet' since it vents back into the intake? Can the stock BOV even fail?

 

with the stock airbox you shouldn't hear the BOV at all.

 

If you have a vacuum pump the BOV is easy to check, as per the instructions in the FSM Fuel chapter page 4B-50.