The way understand it is, like what's been said before, when the throttle butterfly closes(when changing in a manual) the compressor wheel continues to spin and produces pressure. When this happens the air has nowhere to go and causes the BOV to open and close fairly rapidly venting it back into the intake. This flutter sound is then heard back through the intake/air filter. Cars with a pod type filter will have a much more apparent sound. I also think it is possible to have a wastegate type flutter when the ecu signals the wastegate to open and close. I'm reasonably sure the wastegate is a simple open/close setup and doesn't have a variable/anologue operation therefore if you are heating up the tyres at the start of a run then the gate will need to open and close rapidly as your boost pressure bounces above and below the maximum boost setting. I think the BOV flutter is the one heard mostly on the T.