Originally Posted by TopgunZ

It is absolutely the BOV being under the right conditions to allow it to fluctuate between open and closed. When that condition is just right it is not under enough vacuum to be pegged open and the air the SC is making (which it is always making) has to escape. The valve gets pushed up by the air, the spring pushes it back down, due to not enough vacuum. This happens repeatedly and quickly giving that chopping/fluttering sound. I experimented forever with this on my initial kit back in 2014. Vacuum changes from car to car and from environments, namely altitude.

If you get flutter then try a slighlty stronger spring, or you can throw in a shim, like a really thick washer. This makes the spring stronger. Also, adding resistance to the output will help. Place a filter on it (Return style only). I bet if you bought the cover that makes the Q50 return style, it would be gone.

As long as the BOV is staying open in idle and decell, and shut during boost anything goes. Maybe the 3psi spring would work better.