intermittant unexplained 'ding-dong' chime on macbook

bought a macbook a few months ago and i notice that every once in a while it makes a 'ding-dong' sound but doesn't explain what it means. it might be related to battery power perhaps, as i think it only does it when the laptop's unplugged. it doesn't do it when the battery power is particularly low, just now and then. who knows what it is? 🙂