It sounded like white noise at first. When they played the audio reconstruction with the horses, I had to put my mind into that situation to appreciate it. Otherwise it just sounds like a beat up old truck with a squeaky belt.
I had a VW that had a compressor belt that would occasionally squeal. Every so often it would give off this high-pitched screech, and people's heads would turn to see what was making the noise. My son said it sounded like the alien Queen in the movie "Alien."