What is that noise and where the heck is it coming from? The mysterious sound was captured on a doorbell cam in Salt Lake City, Utah at 5 a.m. Neighbors called 911, but no answers were found, though several theories were floated online. Some said it sounded like a windmill, a mild mongoose, or even an alien invasion! But in fact, it turned out to be a slackline, a thin piece of nylon used as a portable tightrope between trees, that was whipping in an intense wind.