Flutter noise :(

I thought the Forge DV was something like £75, not £166!

I didn't think it was that expensive either, I'm hoping it's just an expensive one and other companies will do it cheaper.

Or else I'll flutter on

Ive recently bought a Forge TWINtake and it flutters as you say when boost is built and i let off the throttle quickly. Its simply the natural noise from the turbo. The standard airbox and setup it very restrictive and also dampens noise of the car so that it appeals to people unlike enthusiasts who enjoy hearing whats going on. The TWINtake is not restrictive what so ever so it allows the engine to breath in as much air as it needs, hence you can hear the boost build up. Then when off the throttle from full boost the pressure is released obviously through the DV and noise comes back through the intake.
I would suggest uprating your DV though, ive heard the Rev D valve is much stronger than the original but depending on how much boost your running could mean you still need something stronger.
Im planning on getting to Stage 2+ in the next few months, and ill be using the Rev D DV, untill it cant handle it, but from what ive read it shouldnt have and problems.

Sounds like diverted valve too me matey. Forge item maybe

Ive recently has my car mapped and have exactly the same issue!! spoke to ben at R tech and he says its perfectly normal to get rapid diverter air movement under partial load and turbo flutter under gearchange and rev matching as minor compressor surge occurs which is a common side effect on these but nothing to worry about due to it being so close to the efficiency line on wot, but all exaggerated by the fact you have a free flowing air intake that doesn't baffle the noise!.. all k04 turbos ten to do this and its perfectly fine ! mine sounds like you have an under shield loose and vibrating in the wind on partial throttle but as soon as you put your foot down noise goes clamps hard and off you boost!!