Briggs thumping noise

I recently serviced a 5hp Briggs engine, 128602 0212 E1 on a Snapper walk behind. I removed the cylinder head to clean up a lot of old buildup. The head gasket was in great shape and none of it pealed off so I did not replace it. I replaced the carb, cost was not much more that a rebuild kit. Replaced the fuel lines, installed a fuel cutoff switch, new plug and oil change.

It runs good now but there is a sporadic low thumping sound coming from the engine/exhaust. Not a knocking or tapping but a thumping sound. It is pretty constant but not following any type of pattern or rythm. It does not seem to effect the engine performance or idle but it drives me crazy because something isn't right.

Any ideas? Could that be water in the fuel?

may not be related but make sure the blade is tight. noise travels

Will do, it had a brand new blade on it when I got it, still had the label with it so I didn't mess with it or check it. It cuts great but so it seems unlikely but at least I can rule that out.

The thumping sound is almost definitely coming from the engine itself and sounds more like a compression sound. Would a valve sticking make that sound?