The sound an ice cream machine makes can be described as a low, humming whirr combined with a rhythmic churning or swirling noise. You might represent it phonetically as "rrrrrrr" for the hum, followed by a soft "whir" or "swirl" as the mix is churned. Occasionally, there might be a slight clinking or clattering sound from the machine's components. Overall, it creates a soothing, mechanical melody that signals deliciousness is on the way.
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.