sounds like it may be leaking... but i've never heard of an hks ssq leaking. now i'm not expert but here's my .02- there may be another leak somewhere else in the intake tract. this would explain the bov being quieter, and also the whistle under boost. as far as i know, a good bov should be transparent under boost- it shouldn't be whistling at all until the throttle is shut because any air going through the bov would be NOT going to the manifold, which would indicate a boost leak. it could be as simple as checking all your hoses and tightening clamps.