It’s the movement of air throught the bowels due to the natural contractions of the intestines. These contractions continue even when you are not eating and the nosie is more common if you haven’t eaten for a few hours.
It could grumble or rumble when you are very nervous, as well. It usually happens at the most awkward moments…in church when you are getting married and it’s the quietest point of the mass, or at the dentist, or worse, when you are in the middle of a test in high school and it seems to echo around the room. So embarassing, that boborygm.
I had someone tell me that when your stomach growls, it’s your body using your excess fat to feed itself! I asked if she meant that in a psychological way to convince herself to not leap up to eat something. She insisted that she meant it physically!