Your digestive tract moves food along by a series of muscular contractions that force the contents of the GI tract down the passage and toward their final destination, the rectum. The contents of the GI tract are a mixture of solids, liquids, and gasses (including air that we ingest as we swallow). Whenever the muscles contract, this varied mixture is forced through small passages, making the gurgling noises (these have the poetic name “borborygmi”).
These contractions and noises happen all the time, not just when we’re hungry. But when our stomachs contain food, the noise is dampened somewhat by the mass of the food there. When it’s empty, the stomach acts like a resonating chamber for all of those gurgles, and it sounds much louder.