G'day Jim, I am in OZ, ride amongst other things, a K2 750. My engine has a terrible rattle at idle, up to 1500rpm. But as soon as i pull in the clutch it stops,it is then mechanically very quiet. I have synked the carbs the best i can using my home made water mamometer,but i think it could be sucking a little air around the rubber inlet manifolds. I believe my noise is only the characteristic clutch basker rattle??I am not familiar with other noises of these engines, so you had better get a local expert to have a listen.
Cheers Pablo