A hum is a sound made by producing a wordless tone with the mouth opened or closed, forcing the sound to emerge from the nose. To hum is to produce such a sound, often with a melody.
A hum has a particular timbre (or sound quality), usually a monotone or with slightly varying tones. There are other similar sounds not produced by human singing that are also called hums, such as a sound produced by machinery in operation or by an insect in flight. The hummingbird was named for the sound that bird makes in flight.(Wikipedia)