Name that sound

No clues just listen to it here

I went on a few bus trips. All the trips started on the 30 heading east until i found myself in an unrecognizable place or until I got bored. I would listen for a bit and then either start to wonder on foot or take a different bus. After the first trip I realized a sort of very logical approach to the abstract assignment at hand. This idea I had went sort of like this: if there are things in nature that are unexplainable to humans, but humans can explain everything they make then why am I trying to capture the sounds of man made objects. So I ended up trying to capture nature as it happens. Well I guess that's your only hint is that the sound comes from nature.