It depends on the situation. A bull calling because he's hungry will beller or so-called "moo" like a cow will. When challenging another bull or trying to woo some cows he will make a low, growling or rumbling call, or let out a "battle call" in the form of a bellow. He will also let out a blowing snort in a stance just before he proceeds to fight with another bull, or when warning a human to get out of the pasture or corral, or even when he's in a fearful mode like if he feels cornered.
You write sizzle.
The Colorado beetle.
baaah
ting
BOOM!
bacock
squeak
You could write Bzzzz or Zzzz or any other buzzing sort of sound.
The sound a whistle makes is often described as "whoo" or "wee" depending on the pitch and intensity of the sound.
dribble dribble
"bzzftt"
BWAAAAAAAAAAAAAAAAAAAAAAAAA~!!! At least that's the sound he makes around here.