to groan; to moan; to roar; to howl; to hum (engine, wind, etc.); to buzz; to ooh and aah (in admiration)
groan; moan; growl; roar; hum (of a machine, power lines, etc.); drone (e.g. of an engine)
groan; moan; growl; snarl; hum (e.g. of a motor); buzz