What does the knocking on the wall signify?

Certainly the narrator’s knocking on the wall should be
construed as a mark of his arrogance. That the cat answers from within the walled-off
area might be interpreted by such phrases as “pride goes before a fall” and “murder will
out.” The answer is thus that knocking on the wall is both a mark of arrogance and an
admission of guilt. It is certainly an interpretive remark if a student suggests that he
didn't really hear anything, and the ending is similar to The Tell-Tale
Heart. If a reader thinks that there really was no sound of the cat, it might
be  a stretch to produce any tangible evidence for that, for that would go against the
crux of the story in the style that Poe wrote it.