(a.) Alt. of Monotonical.

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