Digestion in the stomach and intestines occurs through the secretion of digestive juices of their walls. And that digestion occurred more efficient food mass should always be mixed. This is accomplished by contraction of the walls of the stomach and intestines.
Regardless of whether there is food in the stomach or not, every 1-2 hours the muscles of the stomach is reduced and the mucous membrane of the stomach secretes another portion of the gastric juice. If the stomach is empty, the gastric juice and gastric gases and air (which we are constantly gradually swallowed) are moving in the cavity of the stomach and interact with each other, resulting in a purring sound.
Why this no sound when the stomach is not empty? Because the food absorbs it, pressing the contents of the stomach to its walls.
Frequent strong rumbling in the abdomen can be a sign of a dysbacteriosis. In fact, in our gut live bacteria, the combination of which is called the gut flora. Normal microflora necessary for a person: bacteria help us digest food, synthesize vitamins, and are also involved in protecting the body from harmful organisms. But if for some reason the composition of the intestinal microflora changes, beneficial bacteria becomes less and harmful (called pathogens) start hard to reproduce. As a result, the flatulence increases, and this is manifested in a strong rumbling, bloating, pains in the umbilical region. If the roar of a hungry stomach can "calm down", just ate, dysbiosis should be treated.