The Collaborative International Dictionary of English v.0.48:

Sibilate \Sib"i*late\, v. t. & i.
   To pronounce with a hissing sound, like that of the letter s;
   to mark with a character indicating such pronunciation.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sibilate
    v 1: pronounce with an initial sibilant
    2: utter a sibilant
    3: express or utter with a hiss [syn: hiss, sizz, siss,
       sibilate]
    4: make a sharp hissing sound, as if to show disapproval [syn:
       hiss, siss, sizz, sibilate]

Moby Thesaurus II by Grady Ward, 1.0:

84 Moby Thesaurus words for "sibilate":
   assibilate, bark, bawl, bellow, blare, blat, blubber, boom, bray,
   breathe, buzz, cackle, chant, chirp, coo, crow, drawl, effervesce,
   exclaim, fizz, fizzle, flute, gasp, growl, grunt, hiss, hush, keen,
   lilt, lisp, mumble, murmur, mutter, pant, pipe, roar, rumble,
   scream, screech, shriek, shush, sigh, sing, siss, sizz, sizzle,
   snap, snarl, sneeze, sniff, sniffle, snore, snort, snuff, snuffle,
   sob, spit, splutter, sputter, squall, squash, squawk, squeal,
   squelch, squish, swish, thunder, trumpet, twang, wail, warble,
   wheeze, whine, whish, whisk, whisper, whistle, whiz, whoosh, yap,
   yawp, yell, yelp, zip