the chattering/fluttering noise is air coming back out the turbo inlet when coming off throttle.

a dumpvalve will reduce if not remove that noise.

only reason some people think that a DV does it is when they fit aftermarket dumpers that have an overly strong spring in it, and that means not enough air comes out the dumper, so lots more come out the turbo inlet than before, and hence the chattering noise.

trust me, been there, done this, 6-7 years ago up until today, as have countless others here.