this happened to me when I had one of the accessibility settings on. Even though there were no new notifications, it would continue at the time interval I had set.... For emails in my inbox that I'd re-marked unread! Crazy annoying since I always have some emails set at unread.
The setting is under device, accessibility, notification reminder. I thought it would only apply to system notifications, but not so. So every ten minutes my tablet would chirp the entire time it was on. Almost went batty.