"Quote from Xspeed
"The 'flutter' noise reported by many excited enthusiasts may sound good but is actually very unhealthy for the turbo.
The 'flutter' occurs when, upon shutting the throttle, compressed air is caught between the free-spinning turbo and the closed throttle. The consequent back pressure forces the air back through the turbine blades. Thie process, more commonly known as a Compressor Surge or Cavitation, places enormous loads on the turbo and can lead to premature wear of the thrust bearings or, in extreme cases breakage of the compressor wheel. Another common side-effect of the compressor surge is the dreaded turbo lag between gear changes.
The compressor surge or 'flutter' can be fixed by either fitting a blow-off valve onto a system that hasnt got one, or making sure the blow-off valve is matched to the output of the turbo and is not working outside its flow capacity. It is also important to ensure that the blow-off valve is set up correctly."
But yeah you get people saying its not true then you get people saying it is, I find it pretty reasonable tbh.