My late 2008 macbook intermittently makes a gentle "ding" sound. By gentle I mean it's a sound effect, not the kind of beep you get from about to die hardware. I cannot work out at all what is causing it. Sometimes it doesn't happen for weeks then will happen several times in a single day. It happens when I am not doing anything at all (ie. not even pressing keys). I've checked all the sound effects that you can choose in system preferences and it's not the same sound as any of them. I don't use Mail or any other email clients but just in case I checked once when I heard the ding - and I didn't have any new mail. I've also checked my dashboard, and as far as I can see the calendar, clock, weather and calculator widgets don't seem to be set up to make some random alarm sound. Any ideas?