The Sound That Isn't There

Premise of a horror film I thought of while staying up too late.
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Imagine every night you hear the loud humming of what you think are street sweepers driving by.  One night, you get tired of it and venture out into the streets to complain, except there's nothing there.  You run towards the sound, but it just keeps getting louder and louder, with nothing in sight for as far you can see.  The sound eventually starts moving away and goes quiet, like it does every night, and you go back home confused.

The next night, you run out again when you hear the sound, trying to get to the center of it before it goes away again.  This time, you keep running towards it as it tries to get away from you.  At first, you are barely able to keep up with the sound as it is moving away.  At some point, you realize the sound is suddenly getting louder and louder than it ever has before.  You get so caught up running towards the sound that you don't realize it has taken you to a darker and darker part of the city.  Suddenly you realize you are surrounded by complete darkness with no sense of where you are or how to get back.  And the sound just keeps getting louder, despite you not being able to see anything at all, despite you now no longer moving.

And then, in the pitch darkness, you feel something brush past you.