I've never had my macbook air make this noise before, though I've had it about a year now. It's making a noise like when you have Finder open to upload something but you click on another tab and it won't let you. It's a sort of "Bop" sound, not very loud, and has happened even after force quitting things, restarting the computer, etc. It happens very randomly, but very often and is becoming very annoying. It doesn't matter if my headphones are in or not, it still does this. Is there a way to stop this?