[He's watching intently as it makes the sound, trying to confirm if it really is coming from inside the Kricketune, or if there's some exterior apparatus that's moving to produce the vibrations.]

THERE, THAT'S IT. IT'S LIKE A GUITAR — THE BELLY IS HOLLOW TO LET THE SOUND RESONATE. IT GETS TALLER, THE SHAPE OF THE HOLLOW CHANGES, THE SOUND CHANGES.

[Well, in theory, anyway. Because what is Pokemon logic.]