take a guess at odd engine noise?

TrailBeast said
Yep, but it could also be confused with once for a lot of people I think.

Yes, I am definitely estimating things here - I didn't use a stop watch and count the number of taps/ticks. I can definitely say it's not once per revolution.

brian6pac said
wouldn't that be once per 2 revolutions

a wrist pin is faster knock

My apologies - I meant once every 2 revolutions (at least, that's my estimate). Original post has been corrected.