Could just be a loose head but could be a rod/piston issue. Check your valve adjustment and do a compression test (while your'e in there see if the head nuts you can get at are tight), get an oil pressure gauge if you don't already have one and see what pressures it's running (princess auto has cheapo gauges you can screw into the sender hole to get a rough reading), Look around under the motor where the heads meet the barrels and where the barrels meet the case, any massive oil leaks? shiny spots? dark black spots? Pull off one spark plug wire at a time and see if the noise is still there if you rev it. Drain the oil and look it over for metal chunks, screen too.
