I put one shim in the valve the other night, hasn't made any difference - still seems to give a fluttering sound when coming on full boost, particularly in 4th. I have another one I can try but what I'm thinking is that cos my valve is not in reverse, how can it be the valve getting pushed open when the boost is hitting the side of the piston instead of the bottom?
May undo the N249 bypass at the weekend and see if that helps.