Does the wastegate open when releasing the acclerator? Reason I ask is that there is a a wierd sound whenever I change gear or ease off on the power. Sort of fluttering sound. Even at low revs when the boost is <0 it is slightly noticable. It's not the dumpvalve or wastegate solenoid so can only think thta it is something to do with the wastegate or actuator.

 

No, wastegate opens when a predetermined amount of boost is generated thus relieving excess boost generated and maintaining a maximum safe pressure.

This is the dump valve. What model year is the car? All impreza turbos have a recirculating dump valve fitted to the engine which can still be heard, especially with an induction or free flow air filter. This can be substituted for an atmospheric unit to give the audible Pssssst at gear change / release of throttle
Yep, still the dumper
Why?