OK. I have drained the oil and pulled the oil pan off. I found metal shavings in the pan. Before when I started it, it was hard starting with white/greyish smoke coming out of smokestack. It would start, and I could move the tractor, but didnt do much as I knew something was wrong. The knock was present when it was running. Are the metal shavings from a piston ring? I see no apparent damage while inspecting from underneath. Whats my next step in this process. Pull the head off? split tractor? I am not sure I want to tackle this. any input is appreciated. Thank-you