From page 284 of Waterfowl of North America, Europe & Asia:
The Mute Swan is not mute at all. Its most frequent call is a wee-rrrr or wiingrr-iew with a high-pitched second syllable. It will also make an in-rrr sound accompanied with strong hissing in aggression. In flight, the wings produce a whistling sound, typical of the species.
This is the ultimate guide for anyone who wants to identify the ducks, geese, and swans of North America, Europe, and Asia. With 72 stunning color plates (that include more than 920 drawings), over 650 superb photos, and in-depth descriptions, this book brings together the most current information on 84 species of Eurasian and North American waterfowl, and on more than 100 hybrids. The guide delves into taxonomy, identification features, determination of age and sex, geographic variations, measurements, voice, molt, and hybridization. In addition, the status of each species is treated with up-to-date details on distribution, population size, habitats, and life cycle. Color plates and photos are accompanied by informative captions and 85 distribution maps are also provided. Taken together, this is an unrivaled, must-have reference for any birder with an interest in the world’s waterfowl.