Why does my iphone keep vibrating for no reason?

My iPhone keeps vibrating like this: bzzz bzzz. I've no notifications or texts coming in so it's not that. Anyone know how to fix this or why it's doing this?

Are any apps getting updated? Maybe its notifying you of a finished update

Your battery might be low.

Battery may be low
Apps may be updating
Apps may complete updating
You may get inner notifications or banners or badges
You can't see inner notifications, banners from games and badges from apps
may be you have received a message from messenger