Low sound (6)
(Other definitions for rumble that I've seen before include "gang fight in New York City?" , "Heavy vibration; US gang fight" , "Sound of distant thunder" , "Distant-traffic sound; (US) fight" , "Discover the truth about" .)
'low sound' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for moo that I've seen before include "Cartoon sound of a cow" , "Cow's low" , "Guernsey noise?" , "Irritating woman" , "Low (of cattle)" .)