on my mates s2 it ran 19psi i thought the chatter was coming from the air flow meter when the flap opened to let more air in and as the throttle is relased to change gear the flap it hits against the top of the unit causing the sound.
but really what it should be is the waste gate in the turbo opening and closing as the actuator is releasing the unwanted pressure.
rally cars use an extenal waste gate hence the long chatter as they tend to relase the pressure slowly as it is a piston type device probally worked off a spring which vents the fumes down a seperate pipe.