2008 wrx hatch 2.5. Does the standard re circ valve blow off ?
I have got a whistle on boost and a blow off on throttle release.
I'm thinking the re circ pipe to turbo intake may have a crack in it. These cars have the later plastic manifold and plastic pipes.
Car is totally standard with snorkel etc .

 

Make sure the airbox is clipped together correctly as that will cause a similar noise.

 

Found the problem. ! The jubilee clip on the turbo intake hose wont nip up , it jumps back a couple of teeth if you try to tighten it. Time for a new clip.