os 91 fx knock/ping
David,
Many engines .91-sized and larger make a 'knocking' sound at low- to medium-throttle settings. We've investigated a lot of engines that make this sound, and could find nothing physical that could make the sound, and found no engines damaged as a result of it. We've concluded that it's an exhaust note, and can change with fuel type, exhaust system, propeller, and so on. Usually, it's just something to live with.