Why is my phone beeping randomly?

[AC Question]

I'm not getting notifications. I might be playing a game or browsing the Internet, or even just on the home screen when my phone will beep a couple of times. It doesn't happen extremely often, just about once a day, and has been occurring for less than a week. It's not frequent enough to be an annoyance but I'm worried there's something wrong with my phone. Any idea what it might be? My sister's phone also started doing it recently, on the same day as mine.

 

[B. Diddy]

Welcome to Android Central! Which phone? Is it the default notification tone you're hearing, or is it completely different?