Chevy, Silverado, 1996, 5.7, 200,000

This could be a few things. First when was the last time you changed the oil? What is the weight of the oil? Is the oil low?
If any of the above questions offend you then I'm sorry I have to ask. If everything is good with the oil, then take the valve covers off and check for a loose rocker arm. I would re-adjust the rockers to spec. Because of the high mileage. If you still have a knocking sound then it could be a sticking lifter which requires the removal of the lower intake manifold to fix. Or It could be low oil pressure from a worn pump, which should be addressed A.S.A.P.
An engine with 200000 miles on it will naturally have some knocking from natural wear and tear.
If after re-adjusting the rocker arms you still have a knock I would switch to a full-synthetic 10w30 oil or if it's cold 5w30. If you still cannot stand for your engine knocking a little then I suggest re-building the entire upper half of the engine. It all depends on how far you want to go. Let me know what you find and I will help you, if you want, with anything you may need and however far you want to take this knocking condition--from a simple oil type change to a complete tear down and re-build of your engine--no problem. Good luck.

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