I used to be eble to make that farting sound by putting my hand in my armpit and dropping my elbow. It was a real art form where I grew up. So much so that we really looked up to people who had great skill at it. The ultimate challenge was to do it in class and get in trouble for it. One would then have the admiration of one's peers for minutes--if not hours.
I recall a fellow I shall call Earl--that was his name; thus that is the reason I shall call him that--who got in trouble for REALLY farting in class. Turned out it wasn't a fart--or it was, but there was baggage with it. Earl was the envy of all in a weird kinda way. We all liked him until the day in winter when he sneezed into his hand and then chased all of us around the school yard, threatening to wipe his hand on our jackets.