[n] a complaint uttered in a low and indistinct tone
[n] a loud low dull continuous noise; "they heard the rumbling of thunder"
[v] complain quietly
[v] show one's unhappiness or critical attitude; "He scolded about anything that he thought was wrong"; "We grumbled about the increased work load"
[v] to utter or emit low dull rumbling sounds; "he grumbled a rude response."; "Stones grumbled down the cliff"
[v] make a low noise, as of thunder
croak, gnarl, grouch, growl, grumbling, grumbling, murmur, murmur, murmuring, mutter, mutter, muttering, rumble, rumble, rumble, rumbling, scold