[PilotJoker]

I'm using an HTC One M8, and a few days ago I started getting what seemed like random phone vibrations with no notification associated with them, namely while the phone is on Silent or Vibrate (I do this a lot at work). Then I noticed that these random vibrations are coming up whenever I get a text or an email. A small message box pops up on the bottom half of the screen that will say something like "Sms received from null. Ringer volume is too low. Notification volume is too low." I typically use Hangouts for my texts and K-9 Mail for email, so I've gone into the apps and disabled notifications for both the default Messages app and both Mail/Gmail apps. I'm still getting these vibrations and strange notifications about my ringer and notification volume. Any other suggestions as to how I can fix this? Many thanks.