The fact that it stops when you disconnect the spark plug wire really makes me lean towards an exhaust leak. Even a bad rod bearing would keep making noise after you disconnected the plug because there is still compression and vacuum creating loads on the bearing. Valves, lifters and such are all still moving up and down even if the cylinder isn't firing, so that leads me away from lifters and such. However, on any of my engines, when I pull a plug they start running rough and making all kinds of noises. These noises can drown out a quiet ticking, so it may still be there and you just can't hear it.