I heard this just this morning! I wasn’t even aware it was a thing until today. I was walking home from dropping my kids off at school and heard what sounded like a stupidly low flying airplane I looked up but there was nothing! It was a perfectly clear and crisp morning, not a cloud or airplane in the sky. The sound moved slowly too, like it was rolling lazily across the sky. It seemed to hover directly over me for about a minute then it slowly rolled on. It was amazing and frightening at the same time.