My iMac has started played a strange sound at random times and I'm at a loss as to what it might be. The sound is a 'bing bong' and doesn't correspond to any sounds listed under System Pref. I'm not sure what it could be and I can't find a way of listing all 'processes' that are currently running. I can only assume it's an alarm of some sort, but all I can see running from Dock is Safari and Finder. Can anyone shed a little light, please ?