Probably carbon or something trapped between the valve and seat. Tapping on the end of the valve like AJ mentioned might do it. You can also try spinning the valve. You can usually do it by hand with a little inward pressure against the spring. If it was a broken ring or hole in the piston, you would have noticed quite a bit of oil smoke coming out of the exhaust, so, I doubt that's the problem. If the oil pressure was good (at full operating temp), and everything was quiet, you should be good. Since the engine is out, I think I'd still pull the pan and take a look at the bearings and timing chain. A fresh re-seal and new freeze plugs will save you grief down the road.