The noise gas makes as it passes through your intestinal tract on it's way to your anus. Prefarts are most common after eating Mexican cuisine, brocolli, trail mix or cheap beer. Prefarts are oftentimes noisier than the actual fart they will eventually mature into, however unlike a fart --which you can soften the sound of by discreetly seperating your butt cheeks-- there is no way to manipulate your body to soften the deafening noise of the prefart. Most people will try to pass it off as their stomach growling, but since the sound emanates from a decidely lower point on your torso, no one is fooled and you would be better off to point at the dog or an old person nearby.
Must've been a Pre-fart....Stop pointing at my dog.