Mine always flutters, unless I'm really boosting. Just normally driving around it'll flutter everythime I let off the gas, unless I'm shifting at 4K+ rpm. When I boost, 7K+ rpm the car is so loud, when I shift I don't even hear the air being blown off, but there is defenitly no flutter. I have a High boost Aftermarket BOV so I believe in my case it is fine, for it to sound like this.

 

^^ me too!! have a high pitched sound which is the HKS sequential BOV and a flutter (sounds mean by the way) which is the compressor i imagine... I have a HKS Air filter no cold box... and my screw is up semi tight and ive had no drama's with my turbo runnin 1.5bar for the last year.......