How to know why I get sound notifications? Every hour I get 2 notifications (sound) at 27' and 57' exactly. There is no app nor message in the status bar, just the sound. 

It started a couple of days ago. I did not install any new software before that. 

How to know which daemon or app does this ?
It was a mail account failing to retrieve emails. I had to delete the mail account and reconfigure it. There was an icon flashing at the time of sound but not staying in the status bar.