It is normal.
The noises in flatulence are caused by the vibration of the anal sphincter and the expulsion of gas through the rectum. The pitch and volume of the noise can vary depending on the amount of gas present and the tightness of the sphincter.
Random Expulsion of Gas
There is no medical term for that phenomenon.
Fart Gas
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Put a cork in your anus to stop the gas flow.
When? Whenever it can. From where? Mouth, anus, and skin. Lung.
Gas is passed this way through the anus.
Nitrogen
It is from you having passed gas earlier and there are fecal particles stuck on the inner rim of your anus.
A Caucasian newborn's anal area looks just like the rest of the skin. Over time, fecal bowel movements and expulsion of gas continue to permanently stain the skin so the area becomes darker than nearby skin. Obviously, this staining looks darker in dark skinned people.
The least prominent gas in the atmosphere is xenon. It represents no more than one percent on the air in the atmosphere.