Could be friends logging in or out. There's a chime/ding type sound for that. Under options, slide the 2nd, 3rd, and 4th volume bars all the way to the left. You should only hear your game play (balls hit, cup, etc).
If you're playing with others, the chat box might make a sound. And, friend, game and CC invites will constantly make that noise, too. But, with the 3 sound-sliders off, you won't hear any of those sounds, either.