Those muscle contractions are a digestive mechanism called peristalsis, and they're what help your food travel all the way through the gastrointestinal system. After a meal moves through the esophagus, it sits in your stomach where it's churned up and broken down with digestive acids. Next, it heads into the small intestine, which adds enzymes that help your body absorb the nutrients, and then it moves on to the large intestine.
Peristalsis happens in every one of these organs, just in different lengths and frequencies: in the esophagus, the contractions come in one long wave that lasts about nine seconds; in the small intestine, it happens more randomly, a few inches here, a few feet there, at about 8–10 contractions per minute. Peristalsis in the stomach depends on whether it's full or empty. When it's empty, contractions happen about three times per minute, and usually multiple contractions occur at different places in the stomach. When it's full, those contractions get softer and slower, but don't stop altogether—that food has to leave the stomach eventually, after all. That's one reason you don't hear as much grumbling when you're full. Another reason? The same as why tapping on an empty tin can makes more noise than a full one: food muffles the sound, but a stomach full of nothing but air helps the sound travel. Learn more about the science of hunger and digestion in the videos below.