bell sound during log-in and occasionally on the hour

I've been struggling with a bell sound on my macbook air. I can't identify the cause. There's no accompanying notification, just the sound of the bell. Occasionally happens when I log-in, also sometimes on the hour or half hour.

I thought it might be a calendar notification or something, but I can't seem to find one. Anyone know how to check for what's making sounds and why, or default to no notification noises, if that's indeed what is going on?