I, too, hate the whistle hitsound, and have hated it ever since osu first came out.

You can change the whistle hitsound by going to

/private/var/stash/Applications/Osu.app/skins/default

in either an SSH app, or in iFunBox, and replacing the "soft-hitwhistle.wav" and "normal-hitwhistle.wav" with something else.

The whistle is something you can enable while in edit mode to a certain note. When you hit it, it makes a whistle.

Oh, I was making that sound. Sorry, I'll stop.

That noise was just created to piss you off ^_^