bad big end rod bearing. I was lucky and it didnt spin. was just out of tolerance. Mine happened on a dyno run.. fix it ASAP to avoid further damage. depending how skilled you are its an easy job. I have heard of people doing it with engine in the car, I explored this option when I did mine and quickly gave up. MUCH easier to pull the engine and do it on a stand. When I did mine all I did was slap new bearings in, I miced everything then plastigauged each one before final assembly to be sure they were all in spec. Search for how to determine proper bearing size. I am 4000miles post repair with no issues. Good luck with the repair.