Source : Webster's Revised Unabridged Dictionary (1913)

Growl \Growl\, v. i. [imp. & p. p. {Growled}; p. pr. & vb. e.
   {Growling}.] [D. grollen to grunt, murmur, be angry; akin to
   G. grollen to be angry.]
   To utter a deep guttural sound, sa an angry dog; to give
   forth an angry, grumbling sound. --Gay.

Source : WordNet®

growling
     n 1: a gruff or angry utterance (suggestive of the growling of an
          animal)
     2: the sound of growling (as made by animals) [syn: {growl}]