From my memory, that is the sound the game would make on coin drop. So, if there is a mod to the start button that somehow feeds the coin switch, that is the sound you are hearing.

From what I remember, Gottlieb expanded its sounds by using a switch on the game over relay going to the sound board. So, in Game Over/Attract mode (relay off), the game made one set of sounds (coin drop, attract sound, and maybe game start). Once in game play (relay turned on) the same bell chime outputs now signaled a different set of sounds.