What was that sound? A hand-cranked railroad cart that needed oiling? An old firetruck with a broken siren? A group of boy scouts with a dying hippopotamus?
No, no, not a hippopotamus. Too artifactual sounding. Too human-made sounding. More like the siren, like an old fashioned air raid siren. And as I listened, not only did it get louder, but I had the distinct impression that it was getting closer.