The noises commonly associated with flatulence are caused by the vibration of the anal sphincter, and occasionally by the closed buttocks. The sound varies depending on the tightness of the sphincter muscle and velocity of the gas being propelled, as well as other factors such as water and body fat. The auditory pitch (sound) of the flatulence outburst can also be affected by the anal embouchure. Among humans, flatulence occasionally happens accidentally, such as incidentally to coughing or sneezing or during orgasm; on other occasions, flatulence can be voluntarily elicited by tensing the rectum or "bearing down" and subsequently releasing the anal sphincter, resulting in the expulsion of a flatus.
Flatus is brought to the rectum by the same process which causes feces to descend from the large intestine, and may cause a similar feeling of urgency and discomfort. Nerve endings in the rectum usually enable individuals to distinguish between flatus and feces,  although loose stool can confuse the individual, occasionally resulting in accidental defecation, or "sharting".
:D i need to grow up