The unnamed music has been In Grand Theft Auto V ever since September 17, 2013. The reason and cause of this music is unknown, since it only happens on rare occasions. Some even believe this is Brad Snider’s ghost coming back on the player. However, this is far to unlikely, since even Franklin can experience this and he never met Brad in his life.
The most likely explanation for this is if it was an alarm in the distance, or a car alarm. However, the main origin of this remains unknown.