Maybe a broken off gear tooth? I'm wondering if you had engine kick back and a tooth broke off in the starting clutch area. Look at the pics in the starter clutch repair section of the KB. 

  I'm thinking if your having no issues with sounds or vibration I'd just ignore it. It's possible it's something thats  been in there since the bike was made.

Looked through the pictures I could find in the KB and nothing jumped out at me as being the possible source.

Although now that you mention it, the bike did make an unfortunate banging noise a couple years ago when I first started it up after a round of maintenance.  Very well could have been engine kickback.  All I did was rebuild the carb...and perhaps changed the oil?...nothing to do with the starter, valves, engine timing, etc.  Anyway it freaked me out but then the bike has started and run fine since, so.... :shrug:‍??

So your starter clutch area diagnosis is probably right.  Since it doesn't sound like I'm facing impending doom I guess I'll leave it as a mystery until I have to go in and replace the clutch one day.  thanks.

Looks like maybe a piece of the starter clutch.  If I remember there were some little half moon shaped "pins". This kind of goes together the noise you heard when starting that time.