Knocking is never good, especially when the oil pressure drops with it.
If you just picked up the truck, you likely planned on servicing it anyway. How do you feel about dropping the oil pan and checking main and rod bearings? Chances are, you find one or more worn. You can put new ones in from below....
A set of main and connecting rod bearings will set you back around $200.
Also, update the connecting rod bolts while you are in it! The old style (hex) has a habit of leaving its post when not supposed to.
Here is one source for the 12-point bolts you want:
https://www.farmlandtractor.com/i-157326-r401091706-white-oliver-connecting-rod-bolt.html?brand:6
Another:
https://www.jensales.com/products/e...ard-waukesha-zetor-rod-bolt-hercules-478.html
This could be a cheap fix as opposed to engine removal and rebuild (don't get me wrong, it's fun, but it is also a rabbit hole. Ask me how I know).
You can always do a compression or leak down test to see whether the engine would benefit from new rings. That, of course, means heads come off, yada, yada.
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