My problem is fixed. Once I got the piston out and examined it on the
bench in good light, I saw that the bumps on the piston were actually
pieces of the exhaust valve that were embedded in the top. The biggest
one was probably sticking up 1/16 of an inch, overall there were 2 or 3
pieces. Just enough to make noise when running.

Thanks for the advice on the piston; I replaced it with a new one and
new wrist pin, with new bearings and rings. The bore was actually in
good shape, a few passes with a hone cleaned it up.

Put it back together - runs like it should, no noise, oil everyplace it
belongs.

Thank you to everyone who helped me get through this - the truck was
done today and is needed for corn harvest, which we started today as
well.