My little Jack has this sometimes too. He is 1 1/2 years and i have heard this noise 3 times. It sounds like a humans tummy gurgling, except really loud! It didn't seem to bother him, his stools were fine, (I thought tummy upset too), and it lasted about an hour. I never really thought about it once the noise dissapeared. Will ask my Vet about now that you are all mentioning it!