If i'm not wrong, the flutter is commonly caused when the air goes back into your turbo fins.
Without a BOV installed, u'll get the flutter all the time. With a BOV, you will get both the flutter ( at lower RPMs ) and most of the time u'll hear the BOV sound that's IF it's an atmospheric BOV. If it's plumback BOV, u'll hear the flutter too.
Anyone care to input their knowledge?