A wolf whistle is a two-toned sound with a higher, rising pitch for the first half of the whistle, and a lower, decreasing pitch on the second.
There are several ways to put the sound of a wolf whistle on a text:
Hear various ways of wolf whistling here.
You know the Samsung whistle ringtone - five chirpy, chirrupping notes that are a bit like a wolf-whistle? The company claims it sounds 'friendly' and 'trustworthy' but its increasing ubiquity is making it as socially unacceptable as Nokia's signature theme once was. Read more at The Guardian.