I think I've only heard one in my life. It was on surfside beach TX where I lived at the time. I was on a dock and heard a really low thunder sound. I can't say that it was "loud" like it would hurt your ears, but it really rattled everything around me (including my insides). And with that the neighbor's dogs came running over to me with their tails between their legs, and off in the distance I could here a jet really cuttin' through some wind.