I think you're going to have to accept that the exhaust control flaps are noisy, it's not suprising as it must all run in a constant stream of hot dry gas! As far as I know it's only there to control the noise from the exhaust, masking it by making even more racket through an aftermarket slip on de-cat whatever is a waste of $$$ in my opinion. There may well be another problem if the servo that controls the flaps is not working as per the original design, but the exhaust internals are not going to sound slick or silent, they're always going to rattle, have you heard a dry clutch on a Ducati?? Sounds like a bag of nails....but nothing wrong, at least within reason. I'm London based, happy to meet up at Boxhill or whatever to compare rackets, set your mind at rest so you can enjoy your bike. Failing that, visit a dealer or wear earplugs and ignore! As long as it's making power and has a smooth delivery, I wouldn't worry!
Lots of replies thank you very much guys, Okay I'll accept it is normal thanks again,
@zxcv, Yes I'd like to do that please, not too far from me. When are you next at boxhill?