What does 'pinging' sound like?

[cyclecat]

I recently had someone tell me this 'pinging' that everyone talks about is a sound similar to the sound a spark plug makes if you make it spark while not in the engine. Now I know what that little sparking noise is, but what I can't figure out is this - if that is pinging then how do you hear that over all the racket a Nomad engine makes? Awfully small sound to be heard over the lifter noise.