From
http://www.signonsandiego.com/uniontrib/20061224/news_1s24am.html in 2006:
Riffety-Riffety Riff Raff, etc.: Ags don't boo, although they “hiss” bad calls. Really bad calls get this horse's laugh. Should the Cal coach come on the field to argue, he'll hear “Sit down, Bus Driver!”
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The hiss is not an equal subsitute for the boo because IT IS NOT SUPPOSED TO BE.
Booing should never be heard at Kyle. Our players and coaches should never be hissed either, that should be reserved for the refs and poor sportsmanship of opposing teams/coaches.
Likewise, groans and yells of displeasure should be reserved for the coaching staff, not the players on the field, though some in the heat of the moment "why'd you do that" type comments probably can't be helped.