The Collaborative International Dictionary of English v.0.48:

Grumble \Grum"ble\, v. i. [imp. & p. p. Grunbled; p. pr. & vb.
   n. Grumbling.] [Cf. LG. grummeln, grumen, D. grommelen,
   grommen, and F. grommeler, of German origin; cf. W. grwm,
   murmur, grumble, surly. [root]35. Cf. Grum, Grim.]
   1. To murmur or mutter with discontent; to make ill-natured
      complaints in a low voice and a surly manner.
      [1913 Webster]

            L'Avare, not using half his store,
            Still grumbles that he has no more.   --Prior.
      [1913 Webster]

   2. To growl; to snarl in deep tones; as, a lion grumbling
      over his prey.
      [1913 Webster]

   3. To rumble; to make a low, harsh, and heavy sound; to
      mutter; as, the distant thunder grumbles.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grumble \Grum"ble\, v. t.
   To express or utter with grumbling.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Grumble \Grum"ble\, n.
   1. The noise of one that grumbles.
      [1913 Webster]

   2. A grumbling, discontented disposition.
      [1913 Webster]

            A bad case of grumble.                --Mrs. H. H.
                                                  Jackson.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

83 Moby Thesaurus words for "grumble":
   air a grievance, beef, belch, bellyache, bitch, blare, blat, boom,
   booming, brawl, bray, burr, buzz, cackle, cannonade, caw, chirr,
   clamor, clang, clangor, clank, clash, complain, crab, craunch,
   croak, crump, crunch, echo, fret, fret and fume, fuss, gnarl,
   grind, gripe, groan, grouch, grouse, growl, growling, grumbling,
   grunt, hiss, holler, howl, jangle, jar, kick, lodge a complaint,
   moan, murmur, mutter, peal, raise a howl, rasp, reboation, rebound,
   reecho, register a complaint, resound, resounding, reverberation,
   roar, roll, rumble, rumbling, scold, scranch, scrape, scratch,
   scrunch, snap, snarl, snore, spit, squawk, take on, thunder,
   thundering, twang, whine, yap, yelp