Small Engines (Lawn Mowers, etc.): Wheel Horse 520H, wheel horse, onan engines

Question
QUESTION: I have a 1988 Wheel Horse 520H that I have owned since it was new. It has a 20HP Onan engine with just under 600 hours on the tractor. It starts, runs strong, and uses very little oil. Lately I have noticed a sound that I thought was a hole in the muffler. Just to be safe I decided to change the oil and filter. When I was dumping the used oil I noticed very fine metal filings in the pan.
If it runs great, where is the metal coming from?

ANSWER: Are the filings aluminum or steel?

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QUESTION: I did not have a magnet to check. From the looks, I would say it was aluminum. They were bright and shiney like glitter.

Answer
On a new engine it would be normal to see slivers of aluminum.  It's not too troubling to see pieces of steel as that can come from the oil filter.  But seeing aluminum on an older engine may be a sign of a problem soon to come.

This noise that you thought was a hole in the muffler might be the source.  You'll have to isolate the location to id it.  I would run the engine about 20 minutes and change the oil again and look for metal.  Strain it through some fine cheesecloth to catch any pieces.  If there's no more and the noise doesn't seem to be internal, then I'd run it for an hour and change oil again.  If there is more metal and/or the noise is internal, then you have a decision to make.  Personally, if it were mine, at that point I'd be tearing it down and looking for the cause.  But I like the Onan engines.