What causes the stomach to rumble during a crohn's flare up?

Various possibility: The medical term for "stomach rumbling" or intestinal noises is borborygmi. The contractions of the bowels containing fluid and air can produce audible sounds and vibratory sensations in the abdomen. This may be more intense when the bowel is inflamed and/or obstructed or partially obstructed.

Motility: Usually the rumbling occurs when your intestines are trying to move things along. When the intestines aren't absorbing well (during a flare) you are prone to more diarrhea. This increase in movement of fecal material can be perceived as a rumble. Another reason for rumble is if you have a partial bowel obstruction, which is often a high pitched and frequent rumble. See your GI if you are concerned.