The moan is hollow and distant, a mournful, almost mythic tone carried far and wide by the sea breeze. Audible from the craggy shore, it is breathy, irregular, and resonant, a haunting but reassuring aural landmark alerting mariners to Gloucester Harbor. The sound comes from a whistle buoy off the city’s Eastern Point, a bobbing contraption that has guided boats through the rocky depths of Cape Ann since the 19th century.