why do girls make such a loud hissing hisssshhh sound when they pee?

A few days ago, I was going with my aunt's daughter in the car to some gaming spot in Florida. On the way, aunt's daughter said she was holding her pee very badly. Then she got out of the car and started peeing on the side of the road, and I went near her as a joke 🤣. But she didn't realize that I had secretly turned on the camera and was recording her. Anyway, since childhood, I have liked watching girls pee and hearing their 'hiss hiss' sound while peeing. I have many such pee videos of my aunt and her daughter that I secretly recorded at different times in different places. Can someone please answer my question why girls make such a loud 'hiss hiss' sound when they pee?