my mate has an x reg type r.when you start it up and leave it idling it sounds fine, but as soon as you rev it above 2000 rpm it rattles really loudly. sounds like its coming from the bottom of the engine, but not sure. so thinking something to do with the bottom end. any ideas? thanks.

 

Simple thought:
If you mean a light 'tinny' rattle, it could be the heat shield on the down pipe having come loose due to broken welds
Horrid thought:
If the rattle is like two hammer heads being rapidly struck together and brings a tear to your eye then it will be the big end(s) going/gone.

If the latter, and you need some good advice/action free of bull **** call David on www.apiengines.co.uk

911