the way you described your question, to me it doesnt sound like you are talking about the spooling of the turbo itself, it sounds like you are talking about the blow off valve, a blow off valve makes noise when you shift or let off the throttle while the turbo is under load. it dumps the pressure back into the system since subaru uses a recirc valve, but there are others that dump straight to the atmosphere. this sound is what most people think of when they think of a turbo car.