Hi, we use these sort of phrases when we wish to express what is called Onomatopaeia, which is a big word for "sounds" of things. a low and continuous sound, pleasing to the ear, is a hum. You can make this sound by placing you lips together and attempt to say "MMMMM" - that is a hum. It also indicates that the motor, engine or mechanical device, sounds regular and correct. No screetching!
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