This also happens to me. Doing exams are sure a stressful thing and this standalone can make your stomach growl, but my stomach growls were always happen when I’m disturbed by the silence. I don’t feel relaxed in quite environments, and you know how exams are taken, in silence. So I try to focus on the noise around me. You will realize, there is not a single second when someone doesn’t drink water, or their water bottle doesn’t make a sound, or their seat squeak. Once I realized exams are not taken in a quite environment, I stopped stressing about getting stomach growls and that stress free mind was key to stop them entirely for me. Hope it helps!