Here is a small post on notifications in SharePoint online. As you know, when we log in to the SharePoint online portal, we observe (sometimes get irritated too with their creepy sound J ) like this.
If you want to avoid these annoying notifications, do these below settings..
Click on Gear Symbol on Suit bar. Select Office 365 Settings
Select Notifications tab in the page
“Off” the notifications which you don’t want.
Hope this is helpful