Its only like Water Hammer in pipes except your (filter end) pipe is not sealed completely hence the fluttering sound is able to be heard.
Its similar to if you have a small fan in your hand blowing at you, you then blow back at it and you hear a buzzing or very fast fluttering noise, i.e. the blow back from you.
The main reason people put atmospheric BOV's on their car is to release the pressure (during basically the hammer effect) without hitting the blades of the turbo again therefore the turbo spools quicker as its still spinning faster than it would do without. Saying this though, there are a number of arguments that substantiate not having an atmospheric BOV and just sticking with the OE design Recirc valve.
Ant.
