I thought this only happened to me, but my mom's phone started doing it too, and she has a moto g. You know how you can set a watch to alert you every time an hour has passed? It's a quick 4 beeps (beep beep beep beep). My phone will do it at the most random times, there's no pattern to it. What's more odd is that I noticed my mom's phone was doing it too and it does it every time and at the exact same time my phone does it. It even beeps in the middle of calls.