you block the return pipe form the bov, and due to the fact the air can't escape it goes back through the turbine, which is bad for your turbo so i suggest you do not do it! the fluttering sound is called compressor surge! sometimes you can get the flutter sound because your bov isn't tight enough but that is only for aftermarket bov's, this is why vl turbos, the early ca18 silvias make this flutter noise because they never came out with a standard bov this would be the 50 millionth time this has been explained
adrian
p.s:- never thought a 17 yr old would be explaining this to older people