Desert World Whistle:

Towards the end of the Desert World you can obtain another whistle. In the top right corner in the far right hand corner of the world there is a rock directly next to the path. Use the hammer you obtained from one of the Koopa Battles to break it. Once you've found it though you must defeat two fireball spitting Koopas to get it make sure you at least have a mushroom before going into this battle just to be safe.