What is your history with the machine? Is it a sharp knock or more "deep" and internal? Some have mentioned exhaust/manifold leak. I would go with that if it is a sharp tick.
This won't be a popular answer, but my first thought internally was rod bearing. I had an IH engine that did that and it was eating rod bearings. It still had high oil pressure. Those letter series engine make about double the oil pressure an N is set at, so the oil pressure might be a better indicator on the N.
If it does it all the time that means something different than if it is only after an hour of running when the oil is thinned. In cold weather rod knock is usually a little better just because the oil doesn't get as thin when it is 10 degrees outside.
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