Simple:
* Whistle
* Whee-oo
* Woo-woo
* Toot
More detailed:
* Whistleeeooo
* Wheeeeee-ooo
* Woo-woo-woo
* Toot-toot-toot
Onomatopoeia:
* Whew!
* Shrill, piercing whistle
* A high-pitched, piercing sound
Descriptive:
* A sharp, cutting sound
* A shrill, thin sound
* The whistle's high-pitched cry cut through the air.
The best way to write the sound of a whistle depends on the context and the desired effect. Consider the following:
* The type of whistle: A train whistle will sound different from a referee's whistle.
* The distance: A whistle close by will sound louder and more intense than one far away.
* The emotion: Is the whistle a warning, a call for attention, or a celebratory sound?
Ultimately, the most effective way to write the sound of a whistle is to use your imagination and choose the words that best convey the sound and feeling you want to create.