One of the con rod bearings. Turn the crank by hand back, and forth to see if you feel any play or the actual click. If that is not a valve it is the conrod. Have you ever had the cylinder head off? Or more specifically the rocker arms? Did the rockers move freely when you adjusted the valves?
I want to be SURE that is not a valve ticking before telling you if it is the Conrod you will have to rebuild/replace the engine. From the sound of it I wouldn't venture far from home either. Actually I wouldn't ride it. It will break.