Strange unidentified sound playing at random intervals
I would guess that it is coming from Minecraft.  Try playing all the sounds in the ~/Library/Application Support/minecraft/resources/ folder and see if one is what you are hearing.

Be aware that the Library folder in your home folder is hidden by default.  To easily get there, choose the Go>Go to folder menu option (or press shift-command-G) and paste in the ~/Library/Application Support/minecraft/resources/ path.