dagadagadagadagadaga...
You write sizzle.
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.