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Well there you have it. Three opinions and they are all correct. I leaned towards the camshaft bearing because I had a light knock in a C at one time. I pulled the pan and checked all the bearing and plasti-gauged them. When I started again, I still had this light knock. All that is left is the camshaft. But it clearly could be a rod or a main just as well. The sound is different between all three but it is hard to describe a sound. You might try adding a can of STP to your 20W50 motor oil. Oil with STP doesn't drain off so readily. If you can't live with it, start by pulling the pan. You may be able to feel slop in the connecting rods but you may not also. Pull off the bearing caps two at a time and inspect. If one is bad it should be obvious. If they all look good, get plasti-gauge and reset them. Then pull off the mains on at a time and do the same thing. If one is bad, it should be obvious. If they look good, use plasti-gauge to reset these bearings as well. Replace pan and run tractor. If the knock is still there, you'll need to pull the radiator off and the front end and pull the cover for the camshaft. You'll probably need the pan off again to replace cam bearings. This is a lot of work for a small problem. I'd go for the STP trick. I've not found anything else that will work like or as well as STP. Most oil additives do little if any good.
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