Announcing TopgunZ ACE Supercharger Kit!

Optimiser

According to that article isn't that what the BOV prevents (compressor surge)?
Assuming the BOV is working properly and has the correct spring for the application.
My guess is that when the "fluttering" noise occurs, it's from a condition of zero boost /vacuum and the particular BOV's spring allows the valve to open and close a small, fairly rapid amount because in those conditions it's getting some sort of oscillation between tiny boost and tiny vacuum (neutral).