coo

coo
    n 1: the sound made by a pigeon
    v 1: speak softly or lovingly; "The mother who held her baby was
         cooing softly"
    2: cry softly, as of pigeons
    

Coo \Coo\, v. i. [imp. & p. p. {Cooed}; p. pr. & vb. n.
   {Cooing}.]
   1. To make a low repeated cry or sound, like the
      characteristic note of pigeons or doves.
      [1913 Webster]

            The stockdove only through the forest cooes,
            Mournfully hoarse.                    --Thomson.
      [1913 Webster]

   2. To show affection; to act in a loving way. See under
      {Bill}, v. i. "Billing or cooing." --Byron.
      [1913 Webster] Cooey
    

92 Moby Thesaurus words for "coo":
      aspirate, bark, bawl, bellow, blare, blat, blubber, boom, bray,
      breathe, buzz, cackle, call, carol, caw, chant, chatter, cheep,
      chirk, chirp, chirr, chirrup, chitter, chuck, clack, cluck,
      cock-a-doodle-doo, croak, cronk, crow, cuckoo, drawl, drum,
      exclaim, flute, gabble, gaggle, gasp, gobble, growl, grunt, guggle,
      hiss, honk, hoo, hoot, keen, lilt, maffle, mumble, murmur,
      mussitate, mutter, pant, peep, pip, pipe, quack, roar, roll,
      rumble, scold, scream, screech, shriek, sibilate, sigh, sing, snap,
      snarl, snort, sob, squall, squawk, squeal, susurrate, thunder,
      trill, trumpet, twang, tweet, twit, twitter, wail, warble, whine,
      whisper, whistle, yap, yawp, yell, yelp