Pretty recently I've begun to hear a rattle that sounds like it's coming from the engine. It only happens when riding not while idling and mainly when I'm accelerating (never when slowing down and only a little bit when holding a steady speed). It seems to be pretty consistent across the rev range. I thought it could be a low oil level. When I checked the oil it was fine but there seemed to be a hint of smoke escaping from the hole where the dipstick is. I've been putting off my valve adjustment as I'm waiting on a tool I need. Could this be solved through the valve adjustment or is this likely something else? I don't have a helmet cam so there isn't really a way for me to get a clip of the rattle.
Try checking the tightness of the exhaust flange nuts. From your description it may be an exhaust leak.
Macka
Old post, but I identify with this noise, seems to be what I've had suggested could be a loose cam chain.. Did you find a solution to the problem?