The spontaneous and continuous snapping sound occured again this morning. I used a tube to localize the sound and it is coming from the plug area. Regardless, the engine seems to run. I have in the past been guilty of sparking the plug without it being screwed into an engine, but that plane continues to run and fly well. Could someone explain how grounding the plug to the engine instead of just the plug's own metal housing is any safer? The engine isn't grounded any better than anything else. It just has more mass. The return path is still through the hi tension shroud, right??

Ran 1/2 a tank thru the engine...at idle. It quit once, but restarted easily. RPM was around 1800, cylinder head temperature right after stopping was around 260 degs F on the intake side of the head, and around 310 degs F close to the exhaust stack.