An aftermarket BOV is probably a lot tighter than your stock one. Fluttering at low RPM would occur because the blow of valve would not open at lower revs/boost, thus the air going back through the intake (as in going back against the turbines spinning direction, shopping the air), amplified by your FMIC and pod and wellah, flutter for you.
A blow of valve can help effect it, but it doesn't directly make the noise. Forgive me if it doesn't make sense, I need sleep
More info:
http://www.nissansilvia.com/forums/index.p...60&hl=stock