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]

WordNet (r) 3.0 (2006):

grumbling
    adj 1: continuous full and low-pitched throbbing sound; "the
           rumbling rolling sound of thunder" [syn: grumbling,
           rumbling]
    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]