system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5 Why do certain sound frequencies make us feel unnerved/afraid?
gpt
Your brain has a sort of "library" of sounds. For example, loud thuds and booms in nature are usually something like thunder, a large boulder/tree falling, etc. 

These grab your attention because they present imminent danger and you generally have a good idea why. 

For sounds which are not in that "library", your brain has no built in instinct other than to heighten your senses and try to figure it out. The heightened senses will likely make you feel uneasy simply because your brain is trying its best to piece together what's happening and, putting it bluntly, your brain doesn't like not knowing things.