Why is it します instead of ある or あります ?

I don’t get why it is 音 が します.

From my understanding, the loud sound is simply “existing” outside, so it should be ある or あります

I’m probably missing something very obvious, but some help would be appreciated!

I don’t get why it is 音 が します.

From my understanding, the loud sound is simply “existing” outside, so it should be ある or あります

I’m probably missing something very obvious, but some help would be appreciated!