Anyone else getting continuous periodic notification sounds since upgrading to 5.1.1?

[AC Question]

My phone upgraded over the weekend. Ever since, I've been getting my notification sound every 30 seconds or so, with no event prompting the notification. I can't figure out how to turn it off, except for muting all notifications. Anyone else have this problem, and how do I fix it?

 

[B. Diddy]

Welcome to Android Central! Which phone? Is it the same notification sound as all others, or is it very different?