Nouns

(n)rumble, rumbling, grumble, grumblinga loud low dull continuous noise“they heard the rumbling of thunder”
(n)grumble, grumbling, murmur, murmuring, mutter, mutteringa complaint uttered in a low and indistinct tone

Verbs

(v)grouch, grumble, scoldshow one's unhappiness or critical attitude“He scolded about anything that he thought was wrong”, “We grumbled about the increased work load”
(v)murmur, mutter, grumble, croak, gnarlmake complaining remarks or noises under one's breath“she grumbles when she feels overworked”
(v)grumble, growl, rumbleto utter or emit low dull rumbling sounds“he grumbled a rude response”, “Stones grumbled down the cliff”
(v)rumble, grumblemake a low noise“rumbling thunder”