Why do girls make such a loud hissing sound like 'hisssssssss' when they pee?

Let me say upfront that I am actually a 15-year-old boy. Actually, at home I often hear my aunt making a loud hissing sound while peeing. Because when she pees, it makes a very loud hissing noise.
Yesterday, while going to a gaming spot in Sydney with my aunt, a very funny incident happened. While driving, my aunt suddenly said she was holding a very strong urge to pee. Then she drove to the side of the road and started peeing right in front of me. I secretly recorded a video of my aunt peeing from the side, because at that time she was peeing so forcefully and the hissing sound was so loud — I had never seen or heard it this close before in my life.
I had seen her peeing before when she left the bathroom door open, but it had never been as loud and forceful as yesterday. Actually, watching my aunt pee makes my dick hard and I love to cum. I have almost a dozen secretly recorded videos of my aunt and her mother peeing at their house, which I watch while masturbating😋