Because I'm hungry.
Well, the answer is borborygmi. I know it's a crazy word, it kind of sounds like what happens when your stomach growls doesn't it? But, it's actually the name of the process where the walls of your intestines squeeze together to mix and digest food. Now, you will notice I said intestines and not stomach. That is because it really is not your stomach making those noises, it is further downstream after food passes out of your stomach. Sometimes you hear the growling when you're not hungry because your brain automatically turns the digestive system on and off several times a day. But digestion continues whether your stomach is full or empty. You just hear it more clearly and loudly when there is no food there to muffle the sounds.