When people hear their stomachs make noise, most of what they hear is gas and intestinal motility, the normal movement of the intestines. Even when you're not eating, your gut is moving. If you have food in your intestine, it can muffle the sound, but if you have air in there, and that's all you're moving, that is what you'll hear. Some people are more sensitive or have a heightened sense of what's going on inside their bodies, so they hear it or feel it more.
Some people have more gas than others, and there are a variety of reasons for this. One is a common problem that many people don't even realize, which is swallowing air. You can swallow a lot of air into the esophagus and the stomach by eating too quickly or talking while eating, and that can cause belching, bloating or rumbling. You can minimize this by eating more slowly and not talking or exercising while chewing.
Another cause of stomach noise is air produced in the intestinal tract, which is most often due to poor absorption of nutrients. The most common example is lactose intolerance. When lactose in dairy products doesn't get absorbed into the intestine because the enzyme lactase is missing, bacteria instead break it down and liberate gas. When gas is liberated in the intestines, you hear a lot of noise.
Other foods that can cause intestinal noise are fructose, a sugar that can be difficult to absorb, and artificial sweeteners, such as those in sugarless gum or in diet sodas. They contain sugar alcohols that can be difficult for our intestines to absorb; bacteria in the intestines break it down and liberate gas as a byproduct. Many of my patients have cut down on artificial sweeteners, and that has helped to eliminate the problem.
Intestinal noises can be bothersome, uncomfortable and socially awkward, but they're more of a nuisance than anything else. However, there are situations where a serious problem could be present if stomach growling is accompanied by nausea and vomiting. In those cases, you should see a doctor.