What did billy hear that he thought was bad luck?

Billy heard a loud noise that he believed was a sign of bad luck, specifically the sound of a black cat crossing his path. In many cultures, this event is associated with superstition and misfortune. As a result, he felt a sense of dread, thinking it might foreshadow an unfortunate event or outcome in his day. This belief in luck and omens influenced his mood and perspective on what lay ahead.