I get the same thing on my galaxy S7 edge, and even with vibration, but I think is like the phone itself that does that.
I don't remember the vibration with this sound but I have it turned on for my regular notifications.
I mostly get it that way, it randomly vibrates, and when I check, there is nothing new. And it's different from the vibration when you pick it up
Install goodlock and then nice catch. When your hear sound again next time, open nice catch it will show you which app had the last notification.