Actually, I found it much more difficult to try to whistle when I was younger. You know, mouth is still changing, teeth fall out. Way too hard with all going on.

If you can't whistle you can always hum. It's not as close to cockatiel language as whistling is, but it's something.

When I try to whistle to Peach she looks at me like she feels sorry for me as I blow out more air than music.

hahaha!