I've got a 99 Mercruiser 454 MAG MPI with about 350 hrs on it. I changed all the plugs a week ago and ran the boat all weekend without any problems. This weekend it blew the center of a spark plug out (the porcelain) and left the metal in the head. The diode off the porcelain part was missing though. I easily removed the leftover metal (it was all intact) and fished a magnet around inside of the cylinder and also started it without the plug to blow anything out. Once i did this I put a new plug in it and started it. It runs fine but has a taping noise coming from the valves on the same side the spark plug came out of. After getting opinions, alot of people said they think something is hung in the exhaust valve. .....i question that b/c it runs fine. At high rpms the noise tends to go away. (may just be engine noise covering it)..? Other people say possible exhaust leak...bent push rod....bent valve...and so on. Has anyone ever experienced this before??

If you do have a hanging valve with a piece of something as the cause, it is safe to say that piece of whatever is embedded now that you hammered it in place. Head pull is in order imo

Yeah..I was counting on that. If that is the case..hopefully its not embedded in the piston..