Mac Notes Keeps Opening Up every 5 seconds
If you previously had Twonky Media Server installed and removed it, that could be the culprit. I had the same issue, though with Lotus Notes (also conveniently called Notes.app), starting/popping up every 5 seconds.

CleanTwonkyServerData.command (in /Users/Shared) was the culprit, it might be the same for you.