Why does my HTC One M7 have continuous vibrating upon notifications?

[AC Question]

HTC One M7 Continuous Vibrating Upon Notification

Whenever I get a notification such as a text, voicemail, missed call, whatsapp message etc... It will keep vibrating until I look at the message or missed call. It will vibrate every 30-60 seconds until I look at it. It only started happening recently in the past month or so.