The answer to your specific question, found at heptune.com, is as follows:
"The sounds are produced by vibrations of the **** opening. Sounds depend on the velocity of expulsion of the gas and the tightness of the sphincter muscles of the ****. Contrary to a popular misconception, fart noise is not generated by the flapping of the butt cheeks. You can see proof of this in the close-up video footage of Carl Plant's fart on Mate-in-a-State."
For the whole uncensored article named "Facts on Farts" follow the link in the source section.