That horrible screeching sound from the BOV is what I really hate.....
The screeching sound is created when your bov doesn't open fully (or soon enough), and the boost hits the surge line. The air reverts back toward your intake filter and gets chopped up by the compressor blades on the way out and combined with the new material of your inlets, that is making the squealing sound similar to the party toys.
Here is a video, notice how that sound occurs before the plunger opens:
As for the fluttering, that problem is a little more complicated but what is certain is you have a 7+ psi pressure differential between your intake manifold and your chargepipe. If not a hardware issue, that indicates WGDC being too high in my opinion, but I'm no tuner. Being an OTS tune doesn't really debunk that issue, because I see this question come up regularly when people upgrade the inlets specifically. Which tells me the problem already existed but weren't aware of it.