This started a few months ago at grain time, intermittently at first now at every feeding. The horses have feed pans tied to the fence and are on the ground. I just reach thro the fence and drop the grain. One horse started emitting a low deep moan as he lowered his nose into the pan. It almost sounded like "oooohhhhhh, thenk you" in a very deep voice. Now he's moaning as I approach. I've been around many horses and have never heard or heard of a horse doing this. Anyone else?