I have the default notification sounds set to silent but every few minutes a bell sounds. Does anyone know what is causing it?
There are several ways to chase this down.
I would start by checking the notifications bar to see if it will tell you what app is trying to get your attention.
If that doesn't lead you the culprit, take a look through the settings > sound area. You may have inadvertently turned on screen lock sounds or dialing sounds or keyboard sounds.
If neither of those work, head over to settings > accessibility and look through the options for possibilities.