"The bell-beat of their wings..."
See in text (The Wild Swans at Coole)
The description of the swans’ wings as having a “bell-beat” is quite figurative, because wings do not make such a sound. The line is a poetic blending of the bell-like sound of the swans’ cries with the image of the beating of wings through the air. Finally, bells often mark important moments, be it an hour of the day or an event in a religious ceremony. The “bell-beat” here marks the significance of the speaker’s first encounter with the swans nineteen years ago.