How many hours are on the engine, and what kind of maintenance has it had? Were the valves recently adjusted?
If the sound you are referring to is the one that gets more distinct at higher rpm, I'd take an educated guess that it is a valve tappet that is badly out of adjustment so the valve clearance of one valve is way too loose. The jam nut could be loose so the adjusting screw has backed-out, or the lobe on the camshaft is badly worn. That, or the valve lifter that runs on the camshaft is worn. The first thing I'd do is check valve clearance, since that is often the source of the kind of noise that I was able to distinguish from your video. Main bearing, large-end rod bearing, and wrist pin knocks have a much lower-pitched, deeper kind of thumping sound to them - - - Usually.
A simple device that may help you find out what part of the engine is making the noise, is to use a length of 1/2-inch or 3/4-inch plastic water pipe maybe 4 feet long, or if you have some old garden hose, cut off some of that instead - whatever is quickest, easiest, and cheapest. If you use some garden hose, blow through it first to make sure there isn't anything inside it that will block sound (ask me how I know!). Use it like a stethoscope to listen to different parts of the engine, BUT DON'T put the end of it right on the engine, hold the end maybe 6-inches away from it. It takes some practice learning the best way to pick up where specific sounds are coming from. If it is a loose valve tappet, it will be loudest right up on top of the engine at the valve rocker arm cover. The hood and side covers will act as sounding boards, where you might be misled because those will carry sounds from all parts of the tractor - that is where learning how to listen carefully comes in. You will also need to cover one ear with a hearing protector or use a foam earplug.
I hope you find out what is making the noise, from what I can tell, it shouldn't be there. Good luck!
