From WordNet (r) 3.0 (2006):
grumble
n 1: a loud low dull continuous noise; "they heard the rumbling
of thunder" [syn: rumble, rumbling, grumble,
grumbling]
2: a complaint uttered in a low and indistinct tone [syn:
grumble, grumbling, murmur, murmuring, mutter,
muttering]
v 1: show one's unhappiness or critical attitude; "He scolded
about anything that he thought was wrong"; "We grumbled
about the increased work load" [syn: grouch, grumble,
scold]
2: make complaining remarks or noises under one's breath; "she
grumbles when she feels overworked" [syn: murmur, mutter,
grumble, croak, gnarl]
3: to utter or emit low dull rumbling sounds; "he grumbled a
rude response"; "Stones grumbled down the cliff" [syn:
grumble, growl, rumble]
4: make a low noise; "rumbling thunder" [syn: rumble,
grumble]