Both of my Macs play this sonar sound that sounds like "ping ping ping ping" with a small amount of delay / echo. It occurs to me that it is played once a day but I'm not sure why.
I checked iCal but didn't found anything (I don't use iCal anyways but maybe it's connected to Google calendar or my iPhone).
I've heard this sound played by both my MacBook and my iMac but not yet simultaneously.
This sound is not submarine.aiff. It sounds much more like what skub linked to but there are 4 "pings" instead of 1.
It is played at different times (today around 5pm and again at 8.45, but as far as remember not everyday). That's why I'm not sure I could record it, but I could try.
The sound might come from my iPhone, though I'm not sure which apps are alowed to play sound when they are not running. Also I don't see any indication in the message center or something similar.
I think I have to start taking notes on which apps running.