I am just about to pick up my 520EXC, and have been having long conversations with my dealer. He tells me that 2000 models and some 2001 models have an extremely noisy clutch/gearbox from new - apparently due to the clutch bush being made of nylon which wears very quickly. He says that it shouldn't cause too many problems except for the fact that it does sound like it is going to explode !
According to him there is a modification to this bush, which is simply to replace it with a metal bush ( not sure which metal, maybe phospor/bronze ).
later 2001 models should come with this bush in from the factory.
i will be using my 520EXC on the street a fair amount so have been asking in particular about the gearbox, and my dealer knows of no problems at all with any of the RFS models.
Maybe you are just unlucky - i certainly hope so !!
Andy