From what I've just read (thinking back to my Fiat Coupe 20v Turbo days), the chatter comes from a BOV, the flutter comes from blanking it off.
Taken from a comment on YouTube: The subaru's you speak of have lots of compressor surge as the compressed air has no where to vent other than back through the turbo, which chops the air causing that sutututu sound. Wastegate flutter occurs DURING acceleration, completely different
So bearing that in mind, is it healthy to do this at it seems some turbos (from further reading) are not built adequately for this operation.
However, I could be talking out of my arse!