Sorry i don't have an answer for you. I just wanted to let you know your not alone. It happened to me after i creamed my new plug threads. I guess i didn't torque them tight enough and one from the rear bank poped out. It happened at around 100kmh as i downshifted into 4th while coming off the highway. All i heard was pop popopopopopop - metal on metal. Scared the crap out of me. i stopped and shut the car off. My buddy looked at the engine as i cranked it over again. He said the sparkplug was bouncing up and down inside the head. Thank goodness i had my old set of plugs and tools to fix this on the spot. The plug threads were screwed, and fortunately the head wasn't stripped. I wish you luck with finding an answer, and i hope it's not a major pita to fix.
