sibilate \sib"ilate\, 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

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

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