Here is a kinky dinky. I was writing my story and wanted one of my characters to whistle. But as a sound effect, how would you write out whistling? I don't want to go with the lame, "The man started to whistle". I actually kind of wanted it to be they hear the whistling before they see the man. Make it all ominous.
Good rule for describing hard to describe things is to take a more emotional line - how does it affect those around? How does the tune itself sound? Whistling is quite a merry sound, even in grim times it will sound that way, so focus on the impact of the whistle, not the sound itself. Writing onomatopoeia just looks silly most of the time.