Most common cause of that is a loose primary gear nut. You have to pull the right side cover off and see for yourself. Itís the big nut on the crank. Itís left hand thread. If thatís not the knock, then youíre gonna need to be looking at the big end bearing or piston slap, most likely. However, an engine has a lot of moving parts, and there can be a lot of things that cause a reciprocating knock. Iíve even had people panic when a rock gets caught in a skid plate and they think their engine is about to go. If itís the primary gear nut, loctite 243 it, let it sit for 24hrs, then poor oil in it and test it out. I also recommend getting a gear hammer so you can torque the bolt correctly, people make pennies work, also.