How to make specific fart noise

Please don't ask me why I want to learn something embarassing like this, it's a long story...

I want to know how you make that fart noise where you put your hands in a V-shape with the wrists touching together and blow into your mouth. The way I've been doing it for years it squeaks but my friend can do the crackling one, the one sounding like a machine gun. He says you have to blow through the wrists, the middle part, but I can't do it because there's a huge gap.

Can anyone who knows how to do this explain how to do it?