Psychoacoustics

This the 'third' tone, that creates itself.
Here is the explanation, and some great harmonica examples:

http://www.patmissin.com/ffaq/q26.html

And here is a whistling tea kettle. You have to click on the sound icon.
2. Turn mute off
3. Whistle approx the pitch of the kettle
4. Drop the tone of your whistle until you hear
a third tone - wobble that tone about. Odd
isn't it?

https://vine.co/v/bPLX7iLBP7q