IBS can be classified as diarrhea-predominant, constipation-predominant, or alternating D/C. Most people with IBS experience some sort of abdominal pain and excessive gas. The problem is that since we know so little about the syndrome that it's difficult to pinpoint it's cause - it seems to vary so widely among people. They find that some people with IBS have hypo- or hypermotility of the digestive tract, meaning your gastric (stomach) emptying is delayed or too rapid, or some people have decreased or increased peristalsis, which is the term for the contractions that keep things moving along in there. Some people also have an overgrowth of bacteria in the colon, which can cause bloating, flatulence, etc.
The reason people try SSRIs (selective serotonin reuptake inhibitors - antidepressants such as Zoloft, Prozac, etc.) for IBS, for instance, is because your gut muscles have neurotransmitter binding sites and those neurotransmitters can effect the movement of those muscles responsible for peristalsis.
The grumbling/growling you hear is just the effect of movement of food/waste/etc. through your intestine. Not a bad thing in and of itself, just sometimes loud and annoying!
Hope, how is the prep going? Feeling pretty cleared out? What'd you take for it? I'm starting mine today...my GI gave me those pills this time. Weird how I am relieved that I have to take 40 pills instead of drinking that GoLytely crapola. It's all about perspective!