Originally Posted by RCZ

Only partly right. The bov isnt opening at low boost or at all. May be because of the spring or because its stuck shut. The flutter you hear is called surge or boost surge or turbo surge. Its air looking for the path of least resistance back out of the air intake system. This happens to be the turbo. It can cause a lot of pressure on the turbo and can make the impeller on the turbo actually spin backwards or stop very suddenly as the air is going back out. Depending on just how much pressure isnt being released, this can be harmless at low levels or it can be pretty bad at high levels. It makes the fluttering sound because the air is bouncing in and back out of the intake system. When it is a purr, its usually OK. When you hear fewer really loud sorta flutter bounce sounds like the one in the video then you should start to take notice. Although some cars that run really high boost kinda run into this problem if they are using bov's that can leak at high boost because they need to run stiffer springs in the bov. Stiffer springs means leak protection, but it also means that you need to reach higher levels of boost for the vacuum pressure differential to open the bov. In other words, it wont open at lower boost and surge occurs..

I know I couldve made it simpler...