Tractor Repair: Valves knocking, piston slap, piston rod

Question
I ran my Ford 600 for a short time disking up my garden. I noticed a knocking sound in the engine.
I stopped the tractor and changed the oil and filter thinking that was what it needed.
When I started it back up it still has the knocking sound.
The engine had been rebuilt in 2003.

Do you think the valves need to be adjusted?

I read the manual and it said to adjust the valves to .015 with the engine running. Did I understand this correctly or does it mean to only run it long enough to heat the engine up?

Answer
Well, with this particular tractor, Wes, knocking is not good....depending on the knock....adjust your valves while running.....it takes two people, one to run the wrench, and one to run the feeler gauge.
This tractor I am assuming has the factory gasoline engine in in. It was designed for leaded fuel, so lead additive is necessary if you are using unleaded fuel, and you need to use the highest octane fuel you can get or use an octane booster, as well...
As for the knock, it could be carbon build-up from poor fuel, piston slap, or a bad rod. It could also be a bad main bearing making the piston rod knock....since you have 10 years on the rebuild, it is probably time for some major servicing, so pull the oil pan and the head off the engine, check the internal parts and bearings, clean out carbon deposits, and replace any worn parts. It won't take long to do it, and it will be well worth the time. BUT.....do your valves first....you might find a bent valve when you remove the valve cover....