Hi,

I have also heard a metallic knocking sound at a certain RPM. Note: I have two identical engines and I could only get it to happen on one engine.

I found that if I was very careful, I could duplicate it by slowly increasing the throttle. I talked to our local "engine expert" and he told me that this sometimes happens on an engine, and is caused by the resonance in the muffler, and will also happen on OS engines occaisionally. It is usually more noticable when making slow speed passes.

It was difficult for me to believe that a metalic knocking sound could be made by a muffler. So I tried different things and found I could make it go away with a different size prop, shimming the muffler out a bit, or changing mufflers. You may want to give one of these a try to prove it to yourself.

Hope this helps.

Fly4Fun,

Wayne MIller