The growlings in your gut are known as borborygmi among people who like to use technical terms, a cool little word we picked up from the Greek borboryzein ("to rumble").
Borborygmi happen because your stomach is a real workhorse, constantly churning, moving and contracting to move food through the digestive system. When the stomach is full, the food there keeps things quiet. When the stomach is empty, it keeps doing its thing. But without any food in it, the walls of the stomach just rub against each other, which we can both hear and feel. Additional noise comes from digestive juices sloshing around and the gases generated by digestion.
After making its presence known, the stomach will stop growling if you feed it. Even if you don't, it will shut up anyway when the body switches to using stored fat for fuel instead of digested food.