Went to the garage today, taking advantage of the 70 degree weather, with plans to pull out my Walker, wash it off and sharpen the blades, about the last thing I need to do before starting up the mowing season. Well, I started cranking the engine (19 horse Kohler), it fired and started momentarily (cold natured) and something made a noise similar to something hitting a fan blade. I tried again, and the same noise after the engine fired up. I shut it down and opened the body shell, seeing if anything obvious was amiss. Couildn't find anything stuck in a fan blade or belt, so closed the hatch and started it again. No problem. I backed the mower out into the driveway and raised the body again, leaving the engine running. I still couldn't figure out what made the noise, but it seemed to have corrected itself, so I went on about my business, leaving the engine running for about 15 minutes, so the battery would charge completely. Engine ran smoothly, no odd noises. I might note that I changed the oil on the engine just before parking it for the winter, so I was now thinking perhaps the oil took a few seconds to pump up, causing the engine to knock momentarily. The mower set for a couple of hours while I changed the blades and washed it off, and did a few other things in the garage. Closed the shell, and started the engine to pull it in the garage. Same noise. This time, I left it running and lifted the body. The noise was the dipstick, which is vertical on this engine, being blown up against the underside of the body in time with the rpm's. First thing that comes to mind is a broken ring, and/or piston, but if this is the case, why did it not do this for the time it was sitting there running, plus, with my limited experience, I would have thought the engine would have run roughly, which it didn't. Any thoughts by you guys who are mechanics, which I am not? Just for your information, the engine only has about 400 hours on it. I'm just curious what to expect when I take it to the shop. Thanks.