WordNet sense 1 (a loud low dull continuous noise):

rumble, rumbling, grumbling

WordNet sense 2 (a complaint uttered in a low and indistinct tone):

murmur, murmuring, mutter, muttering

WordNet sense 3 (to utter or emit low dull rumbling sounds):

growl

WordNet sense 4 (show one's unhappiness or critical attitude):

grouch, scold

WordNet sense 5 (make complaining remarks or noises under one's breath):

croak, gnarl