Why are some farts silent?

Answer 1

The level of noise a fart makes depends on how much the anus and 'butt cheeks' vibrate when the fart leaves the body, and on the velocity of the gas, basically how fast it's moving.

Farts are made of gas, right? I know they are, I just asked you a question in the middle of my answer to make you feel more included. Anyway, yes, farts are made of gas. Gas, itself, is silent. For instance, if you go outside, and there isn't wind, you cannot hear the air. Same with farts, really. What makes the noise is the vibrations of the anus and the 'butt cheeks' as the fart passes through them. The speed of the fart is also a factor hear, believe it or not. It connects to the vibration aspect. If the fart is moving slowly, it's not going to provoke much vibration. However, if it's moving quickly, it's going to make itself heard!