When a person's stomach growls, the digestive system is undergoing peristalsis, according to Jessika Toothman for HowStuffWorks. These are contractions that push chyme down the gastrointestinal tract. Chyme is a slurry of food, digestive juices and liquids.
Along with the chyme being churned and pushed down the GI tract, there is gas and air, explains Toothman. The growling, or borborygmus, is just the gas and air being squeezed together during this digestive process. Though a growling stomach can happen any time, it is more noticeable when the stomach is empty as food and chyme muffle the sound when the stomach is full.
Since the function of peristalsis is to move food and chyme down the GI tract, it does not seem to make sense for the stomach to growl when there is nothing in it, states Toothman. However, about two hours after it is empty, the stomach sends signals to the brain, which then sends signals to the muscles of the digestive system to begin peristalsis all over again. These contractions move along any remnants of food and also make the person hungry. They continue every hour or so and last from 10 to 20 minutes until the person has something to eat.