I notice after accelerating when I let/ease off the throttle the BPV seems to flutter it's an aftermarket (greddy) it was on the car when I bought it. I noticed it after I put hard intercooler piping on the car. You can hear the air flow build and the BPV open also now. the flutter occurs only after reaching cruising speed, not when accelerating. Normal?

 

so its only fluttering in low boost? whats it sound like in full boost?

 

I had the same problem with my greddy sometimes after letting off at part throttle in higher gears. There is an adjustment screw in the top of the bov. I ended up buying a synapse bov and haven't had any flutter since then

 

Its brief only for a second or two immediately after letting off the throttle. Seems fine otherwise under boost, the car feels powerful I'm guessing it is just as the throttle closes there is still pressure from the turbo on it's way and the valve let's it off???

 

from what it sounds like, it could be the spring on your bov might be too tight, if you loosen it up one turn counter clockwise, and try again that should loosen it up to help with partial throttle, im running a greddy type rs, u gotta find a happy medium between partial throttle flutter and holding boost in the top end...
try it and report back