It does that because of the whole car setup, not just the BOV. It's a combination of the intercooler, piping, turbo, etc etc. And the fact that the HKS Racing Type II BOV is a single stage valve, and only operates under high boost levels. Hence, you get a lot of 'reverberation' through the intercooler and piping.
If you run this BOV on a relatively stock motor, it will sound much like a HKS SSQV. And similarly, if you get a HKS SSQV BOV on a setup with large intercooler and piping, you can get it to flutter.
Fluttering is caused by the pressure finding it easier to escape in places other than the BOV. You have to remember that the outlet for the BOV is only small, and filters and intercoolers provide a LOT of free room when you're blowing off.
As for the whistling sound, that's just how the HKS sounds. So if you want that fluttering whistle, get either HKS SSQV / HKS Racing Type II.
I recommend the SSQV for people running under 15psi, and the Racing Type II for people running over 15psi.