do a search for multi-harmonic singing.

using overtones, you can essentially sing 4 notes at a time. One drone-like bass note, another note an octave up from the bass note, then add a note through your nose which is like a normal singing voice, then add a whistle-like note on the top (though it isnt whistling with the tongue).

Its weird, and makes a sound similar to the bag pipes, but with your mouth!