Assuming the carb is securely mounted and the noise is not from the tappets, there are several things it could be. One is piston slap. Piston slap translates as a worn piston "rattling around" inside the cylinder. Alternate to that, the wrist pin may have worn the connecting rod to the point where that is the problem. Alternate to that, the wrist pin itself may have wallowed out the piston on each end. Broken rings can make noise (but the bike would be smoking). Lastly, the connecting rod may be rattling around on the crankshaft. Change the oil and look for metal particles in the oil. No matter what the problem is, the engine is going to need to be torn down.
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Agree, one way to find out is to unplug the spark plug wire to make it a dead cylinder to see if the noise stops.(that eliminates the pressure on the rod) then you will know if it is the valves or in the top or bottom of the rod.
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