Mine did the exact same thing! It would make the sound every time any sound was made (such as typing or unlocking). I think it might be a bug with an app that caused it.
Mine does that too on certain screens (also when unlocking it) and if a "gray" overlay comes on the screen like when a pop-up menu or a notification appears, but it's not that loud. You have to be in a quiet room with it close to your ear to hear it.