iPhone vibrates several times a day with no visible notification Several times a day for the past several weeks, I'm noticing my phone briefly vibrating as though I've received a notification of some sort. However, upon checking, I have no new notifications of any kind. 

Per the suggestion in How do I know which App is sending Vibrate-only Notification?, I've triple checked my notifications, and any app which has sound enabled (and therefore vibration when phone is muted) should also either badge the icon or appear in history or the lock screen. Neither is occurring. 

Help?! As someone who disables the vast majority of notifications so I can stay focused during the day, this is driving me crazy!
Check your mail notifications specifically: it is possible to have vibration without sound or other notifications on a per account basis.

Settings > Mail > Notifications and then traverse each account. Make sure both sound and vibration are both set to none, if that’s what you want.