groan ②growl ③hum (of a machine, power lines, etc.)

to groan ②to roar ③to sing in a strong, low voice (esp. traditional chant or recitation) ④to be about to burst

groan ②growl ③hum (e.g. of a motor) ④device attached to a kite that makes a humming sound when the kite is flown

to a great extent

to let out a roar (hum, groan, moan)