This is one of those mysteries that I haven't been able to solve. For context, I was hiking Pima Canyon back in 2017 or 2018. I was with another hiker. After about a mile or so, the other person said they needed to take a break, so I decided I would get a quick trail run in. I started jogging further along the path, dodging boulders and cacti.

After about another mile (so I'd guess two miles into the hike from the trailhead), I heard a sound. It was one of those situations where your brain doesn't know how to interpret what it is sensing, so it fills something in as a placeholder. In this instant, I thought it was the sound of someone starting a lawnmower. It was a brief sound -- maybe 1 second in duration. It also sounded close to me.

As soon as I came to the realization that nobody was mowing their lawn out here, I felt very threatened. I bent down and grabbed the largest rock I could find, and turned around and started walking back down the trail. After a few minutes, I picked up the pace and sped back to the other hiker.

To this day, I have no idea what that sound was.