Do you mean loud intestinal noises WITHOUT actually passing gas? This has happened to me too--once during a final exam! (In fact, it just happened yesterday at a friend's house as well.) This happens when I go too long without eating.
From what you posted, it doesn't sound like you're eating enough. Also, you should be eating every couple of hours. If you're still on the BTC diet, try having a piece of toast or some Crispix and some tea before class. The contractions of my intestines seem to be a lot louder when they're empty.