Why does your stomach growl when you are hungry? It is lined with glands that go into action every three or four hours whether or not food is present. Just thinking about food can start the glands expelling digestive juices. These juice will gurgle and rumble (growl!) as your empty stomach churns away.
Your stomach makes about 21/2 quarts of these digestive fluids everyday. These juices are composed of water, hydrochloric acid, and an enzyme that breaks down proteins. This acid will actually kill some of the bacteria you swallow with your food.
Hunger is certainly a factor. When you haven't chowed down in a two hours, receptors in the walls of your stomach trigger the hunger-arousing hormone ghrelin, which tattles to your brain that the pipe is empty. There are other things, like low blood sugar, that also send that message. And your brain, being the boss, steps in to solve that problem by releasing the hormone orexin, which tells your stomach it might be time to fill ‘er up again.
Whether you call it grumbling, rumbling, gurgling or growling, from time to time everybody's belly chimes in. These noises might sound like they should be coming from a noisy pot of bubbling stew rather than your stomach.
I swear I asked this question a week ago to my science teacher and she said as your food begins to digest, the air you breathe began to form in your stomach and your stomach starts to growl from the air.
Though stomach growling is commonly heard and associated with hunger and an absence of food in the stomach, it can occur at any time, on an empty or full stomach. Furthermore, growling doesn't only come from the stomach but, just as often, can be heard coming from the small intestines. Growling is more commonly associated with hunger because it is typically louder when the stomach and intestines are empty and so the organs' contents don't muffle the noise.
This growling has been of interest for so many years that the ancient Greeks came up with the rather interesting name for it: borborygmi (the plural of borborygmus). The etymology of the term relies on onomatopoeia; it is an attempt to put the rumbling sound into words. Borborygmi actually translates as "rumbling."