See if it's the sound set to play under Control Panel -> Hardware and Sound -> "Change system sounds", the 7th from the end of the list - "Start Navigation". One of the first things I do on a new install of Windows is set that "Start Navigation" to "None" at the lower left corner.

If that's the sound, but you prefer to leave it on, it's going to likely be a long process figuring out which process (as noted by @DaveRook above) is "navigating".