Unknown notification sound on macbook (13", retina early 2015)

Hello,

for reasons I don't understand, my Macbook started to make a notification sound I have never heard before. It's not regular but it can be 3 times an hour. I have turned off all the notifications sounds and it plays even when the computer is in «not disturb» mode.

I have read other questions similar to mine but could not find the answer. Battery was changed a bit before that started to happen.

Thanks for your ideas...