The tooth whistle is a mouth position that creates a high pitch screeching noise.

How it's done

1. Do the 'f" sound and prolong it
2. While you're making the 'ffffff' sound, create a small hole between your top teeth and your bottom lip
3. Shift around the position of your bottom lip until you get a small squeak or whistle noise
4. Since this is only a mouth position, you can layer with it humming and throat bass

Video tutorial

Other helpful tutorials

TylaDubya's inward tooth whistle tutorial

Fat Tony's Tutorial