Why do i keep hearing a notification tone when I turned off all notifications and also put my macbook on do not disturb?

First I was hearing a notification tone intermitantly when an email came in; it was never consistent and only started after I'd had my macbook for about 7 weeks. Today, I found the way to turn off all notifications and put the computer on permanent do not disturb in system preferences. Now I'm getting the tone sort of at random. Can anybody tell me what is going on? I have a 2014 macbook pro 15".