Synonyms for Coo:

n.

croak, breathe, mutter, croon, under your breath, hiss, whisper. cheep. cry (noun)
caw, meow, whinny, moo.
ululation (noun)
bray, squawk, chatter, bark, trill, cluck, neigh, squeal, cuckoo, caw, meow, grunt, gobble, moo, yelp, hoot, cackle, purr, snarl, snort, growl, whinny, mew, honk, bleat, twit, cry, ululation, warble, chirp, howl, roar, call, bellow, bawl, bay, crow, quack.

v.

ululate (verb)
bray, squawk, chatter, bark, trill, cluck, neigh, squeal, cuckoo, caw, meow, grunt, gobble, moo, yelp, hoot, cackle, purr, snarl, snort, growl, whinny, mew, honk, bleat, twit, cry, ululate, warble, chirp, howl, roar, call, bellow, bawl, bay, crow, quack.

Other synonyms:

cheep. Other relevant words:
cheep, breathe, hiss, croak, whisper, croon, mutter.

Usage examples for coo

  1. The overseer coo ee'd softly. – A Little Bush Maid by Mary Grant Bruce
  2. The wind sighed softly in the branches of the 'Tavau' trees, from out the green recesses of the 'Toi' came the plaintive coo of the wood- pigeon. – Robert Louis Stevenson a Record, an Estimate, and a Memorial by Alexander H. Japp