Unknown system alert durin unlocking screen.

[WPCentral Question]

Hello, I have got the Nokia Lumia 730 fews ago. I am satisfied with, but two days ago some the System alert started play the sound. Without any text notification. It occurs sometimes per day, during unlocking Lock screen mainly and during calling too. I had turned off System alerts, because it is strange when I am unlocking phone in office and it stars play some melody. Can you help me solve what alert it can be, and what I have to do to turn it off? Or can I have turned off all System alerts all the time?