It's the extra ball "knocker" (speakers) that goes off every time you win an Extra ball (SOL hole, 50M, 200M etc) or get a match after a game.

I've got mine turned off as it was annoying me too! Messes up the audio when it goes off and just *isn't* as good as a proper knocker. 

The setting for extra balls is in Adjustments -> Extra Ball -> Extra Ball Knocker

I don't think you can change the match knocker settings.

HTH

So many knockers - I feel like I'm in a Benny Hill sketch...