Nouns

(n)rumble, rumbling, grumble, grumblinga loud low dull continuous noise“they heard the rumbling of thunder”
(n)rumblea servant's seat (or luggage compartment) in the rear of a carriage
(n)rumble, gang fighta fight between rival gangs of adolescents

Verbs

(v)rumble, grumblemake a low noise“rumbling thunder”
(v)grumble, growl, rumbleto utter or emit low dull rumbling sounds“he grumbled a rude response”, “Stones grumbled down the cliff”