This sound is pretty common in OS engines I think. I think I've heard that in every one of my 5 OS engines thus far. Some people say there's a possibility that it has something to do with the amount of play in the crank shaft, especially when the engine gets warmed up. Last time I actually paid attention to it I heard the sound when letting off throttle, and it would go away with the slightest bit of throttle advance.

I think if it is something to do with the mixture it is a symptom of being on the lean side. A few older experienced guys said they thought the sound was the engine pre-detonating, in which case a colder plug would help.

What plug are you using?

Most people have not been able to figure out the sound, but judging by the amount of cases I've now seen and heard of, ALL with OS engines, I would say it is 'normal'.

Remember that everything you are reading here are just thoughts and ideas. It's hard to diagnose something without actually being there, and most solutions are stumbled upon by experimenting. Try a small change and see if it helps. If not, change it back and try something else. If you are that concerned about it, I would either try a colder plug or a richer mixture.