The Collaborative International Dictionary of English v.0.48:

Growl \Growl\ (groul), v. i. [imp. & p. p. Growled (grould);
   p. pr. & vb. n. Growling.] [D. grollen to grunt, murmur, be
   angry; akin to G. grollen to be angry.]
   To utter a deep guttural sound, as an angry dog; to give
   forth an angry, grumbling sound. --Gay.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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,
       growling]