Sounds like Notifications app.
Maybe some other app. In SystemPreferences->Users&Groups->LoginItems tab: remove all entries with the minus sign underneath. This will not uninstall anything, you have to start those apps by hand now. Restart.
Still there?
Yes: Restart again while holding the shift key, this will start the mac in SafeMode: only the necessary items are loaded to run.
Still there?
Yes: Make an etrecheck list and post it here.
No: Restart, put the items in the LoginItems back, one by one, restart after each entry, and you will find the "culprit"
Please report accordingly. I will be back in 24 hours.