If the dump valve is sticking slightly then the air charge when you shut the throttle to change gear has no where to go but back thru the system..the noise you are most proberbly hearing is the sound of this forced back boost charge being chopped up by the compressor impellor blades. I aint got a clue as to what actuator chatter is? Unless you mean a loose actuator thats rattling or a loose penny washer?? I know you dont mean waste gate chatter cos you said you've got a actuator so its not an externaly gated turbo.