why am I getting notification sounds on my galaxy s7

[Android Central Question]

I am getting a notification sound on my galaxy s7 even though I have notifications turned off for all apps. I have tried muting the phone completely, setting it to do not disturb, restarting the phone, etc. The noise that it is making is not one of the notifications sounds that I can chose from the menu of sounds. And there is no notification that pops up on the screen - even the number of emails on the email icon has not updated. It took me a while to figure out what was causing the noise (new email).

I have had the phone since may2017 - this started in the middle of the night last night.

Any help is appreciated.