Mute swans were named in contrast to noisier relatives such as the whooper and Bewick's swans, whose loud bugling calls carry across kilometres. The mute swan's vocal repertoire is smaller and quieter, but it is not absent: adults hiss when threatened, snort, and use soft grunting and puppy-like notes between mates and to cygnets.
The more distinctive sound is mechanical. In flight the wings produce a strong pulsing hum, often described as a throbbing or singing note, audible over a considerable distance. It appears to function in keeping flying birds in contact and is unique enough that experienced observers identify the species by ear without seeing it.
Two other beliefs deserve correction. A swan cannot break a person's arm with its wing, a claim with no documented case behind it, though a defending bird can strike hard enough to bruise. And the swan song, the idea of a final melody before death, is a classical literary motif rather than an observation.