Last time after I cleaned the spark plugs there was a noise when the engine started, coming from the head at hole 6. Wondered if something bent in the head from revving it up. Didn't drive for a few days. Today put a new plug in #6.  The old plug tested more or less OK, ground side to threads intermittent or bad contact with meter. Did some tune, started the engine, it sounded fine, no noise at #6. What happened ? I guess the sound was a spark jumping but I couldn't see it and the plug looks OK on the outside. It was torqued to spec.

My guess is a loose or cracked insulator.

maybe a fracture in the ceramic?