xj750 Maxim carb #3 makes "tapping" sound up to 3K RPM

The "tapping" sound occurs at takeoff under load and on up to 3K RPM at 30 MPH. Seems to quiet down at a higher rev once speed goes above 3o MPH. Does this sound familiar to anyone here?

My friend recently synchronized the carbs, first adjusting the screws to balance the butterflys while monitoring a mercury meter and then adjusted the pilot screws and then the "tapping" seemed to go away. H0wever, the next day the "tapping" seemed to return at some point during a 30-mile trip on the highway. Ran great! Have since put another 100 miles on it and the "tapping persists" like before. Also seems the noise amplifies through the gas tank, so maybe on a full tank at that point the sound was not so evident? Perhaps during the 30-mile ride there was enough fuel consumed and the tank not so full that the sound became more evident.

During the carb synchronization, we used a stethoscope to pinpoint the origin of the "tapping" sound. While touching the stethoscope probe against both the 3rd carb manifold and the boot connecting the 3rd carb to the airbox, we detected a loud and odd banging noise. Could there be a problem with the carb diaphragm? I checked it beforehand and it seemed fine and seated properly. What about a possible damaged valve?

Engine has 11k miles. While trying to start it there was a very loud "BOOM", as the plug sparked when I turned the ignition back on and obviously there was gas mixture ignited in a cylinder. At that time I had the plug wires pulled on cylinder 1 and 2, but not on 3 and 4.

 

Have checked and adjusted valve clearance ( shim change) ? Clappy valves are happy valves , the valve lifters on these engines should have clearance but you don't want too much. Since you state noise at or around number 3 cylinder the cam chain adjuster could need to be checked or worse the cam chain guide could be starting to break up allowing slop in the chain. The other item to check is the exhaust system could be a leaking and this can sound like a tapping noise.