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. It’s known colloquially as “the groaner,” a term immortalized by T.S. Eliot in a 1941 poem called The Dry Salvages, inspired by his boyhood summers in Gloucester. Eliot writes of “the heaving groaner” as one voice among many in the chorus of the sea. It works like this: Ocean swells lift the buoy, sucking air into a chamber. When the buoy falls, the rush of air escapes through a 155-pound cylindrical whistle, which groans with the exhale. This buoy — there are a handful of similar ones along the Massachusetts coast — has also been called “Mother Ann’s Cow,” a reference to a nearby rock formation resembling a woman keeping watch over the water. Gloucester’s fishermen, residents, and visitors have long heard it lowing in the waves.