WordNet (r) 3.0 (2006):

yammer
    v 1: cry loudly, as of animals; "The coyotes were howling in the
         desert" [syn: howl, wrawl, yammer, yowl]
    2: complain whiningly [syn: whine, grizzle, yammer,
       yawp]

Moby Thesaurus II by Grady Ward, 1.0:

59 Moby Thesaurus words for "yammer":
   babble, bark, battle cry, bawl, bellow, call, caterwaul, chat,
   chatter, cheer, clack, creak, cry, gab, gabble, hail, halloo,
   holler, hollo, hoot, howl, hurrah, jaw, keen, pipe, prattle, pule,
   rallying cry, roar, screak, scream, screech, shout, shriek, shrill,
   skirl, skreigh, squall, squawk, squeak, squeal, ululate, ululation,
   wail, war cry, war whoop, whimper, whine, whistle, whoop, wrawl,
   yak, yap, yawl, yawp, yell, yelp, yo-ho, yowl