It could also be the sound of a monster or a demon that is lurking in the shadows. This creature might use the 'po po po' sound to disorient its victims. Say, it lives in an abandoned place and when someone enters, it starts making the 'po po po' sound to create fear and confusion before it attacks.
The Neverending Story is a well - known fantasy film with a specific set of actors from its time of production. Jack Black is a great actor in his own right, but he was not involved in this classic movie. If you were thinking of someone who looks or acts like Jack Black, it still wasn't him in this movie. This movie has a distinct cast that helps create its unique atmosphere and story.