It does sound a bit "tinny and rattley". Almost sounds like an exhaust leak as the noise is synchronized to the RPMs. Have you looked carefully underneath and behind the pipes on all four mufflers? Checked for bad gasket at the head?
One thing you can do is to try the "mechanic's stethoscope" trick: with motor idling, use a long flat blade screwdriver, handle pressed to your ear, tip on different parts of the engine. It transmits the vibrations pretty darn well and you'll be surprised what you can hear. Move around to different spots on the engine, frame and exhaust to help locate the source. Check the clutch cover, stator, points cover, valve cover, front back, left and right side of the motor until you find the loudest spot.
If the noise is solely internal, then you're going to have look at the chains if the valves are indeed properly adjusted. But you may be surprised-
