birring, bombilation, bombinating, bombination, booming, burring, buzzing, drone, gabble, gibber, gibbering, hum, humming, jabber, jibber, low rumbling, maundering, monotone, monotonic, mouthing, mumble, mumbling, murmuring, mutter, muttering, purring, soniferous, sonorous, sounded, sounding, susurration, thrumming, tonal, toneless, whirring, whisper, whispering, whizzing