To my understanding, the fluttering sound can come from the Wastegate (known on some high hp and high boost GT cars). This is normal for turbo cars, due to boost and how the car release the boost pressure (both intake and exhuast side). Intake side taken care by the BOV and the wastegate controls the boost by regulating the amount of the exhuast gas should go through the turbine (the mechanical/solenoid boost controllers give the feed back from the intake side that boost level is achieved.. the wastegate does all the dirty work).
you can hear the fluttering from a stock wrx setup, by just unbolting the air intake and letting the turbo spool up and slowly letting the throttle off... it's how you let off on the throttle that would trigger it. Basically the turbo is winding down, and the butterfly valve in the throttle body isnt completely closed.. so the air going into the throttle body doesn't create a backpressure for the BOV to go off. At this effect, the ECU reads off that the the throttle is letting off and air intake boost levels are climbing.. this is where the wastegate comes in and just completely let off the boost by slowly leaking it out in a fluttering noise, so that way when you let off the gas, the car slows down, and boost goes down.
If you hear a fluttering coming from the BOV (usually very loud), that is a very bad sign.. the BOV is either busted because the spring has no more static ability to keep the valves closed at normal boost levels, or the wastegate is busted and all the exhust gas is going into the compressor wheel.. meaning your turbo is spinning even at idle and you're not even moving.
Correct me if I am wrong people.
Thanks
Keefe