Blow-off valve doesn't flutter, it makes a 'whoosh' sound

If a car is making a fluttering noise when venting at high rpm and boost, then the BOV isn't doing its job. Contrary to popular belief the noise is actually emitted from the turbo, not the BOV itself.

With an aftermarket BOV fitted some non-detrimental low rpm fluttering does characterise certain cars (as mentioned above) particularly where a larger turbo and intercooler have been fitted.

When properly adjusted, the valve should make a clean PSSSH or Whoosh sound when venting a significant amount of boost. If you want your BOV to flutter a bit, try increasing the spring preload by turning the adjustment clockwise. It's perfectly safe to set your BOV up to cause some low-rpm flutter, as long as it vents with a whoosh at high-rpm and boost.