Loud horn-like noise for thirty minutes every few minutes - didn't notice a pattern.Loud sound like an ocean liner in the middle of town, hundreds of miles from beach here. The sound was so loud my neighbor went outside to investigate it, too. My daughter and I recorded it on a mobile device. We also drove in the direction of where the suspected sound originated believing it was a nearby event, (tuba players in unison??) however no such event involving loud horns or alarms was occurring in any direction at the school stadium or at our local fire dept. Also noted a man in the street looking in the same suspected area while talking on the telephone about what he had heard. After thirty minutes, it had stopped. It may be nothing, but I had not experienced any sound like it before now and felt obligated to report it. The only proof I have is an audio recording in which the sound is clearly audible a few times. This was not a train, nor a truck braking, nor any other usual "city" noise. This was an unusual phenomenon and unlike anything I've heard.