In the play "Macbeth" why is the insistent knocking of Act 2, sc. 2, dramatically effective?

The knocking at the gate of Macbeth's castle is effective
because it occurs just after the murder of Duncan when Macbeth and Lady Macbeth are both
in a highly anxious state of mind.  The two, having just killed their king, do not have
the luxury of relaxing and calming themselves.  Macduff is knocking at their gate
because the king had asked him to arrive early that morning and waken him.  Now Macbeth
and Lady Macbeth must hurriedly wash their hands and, as Lady Macbeth commanded, get
their nightclothes on so that it appears they've been awakened from their sleep.  In
reality, they haven't been to sleep yet because of the murder.  Macbeth is so
distraught, though, that he declares he cannot return to the king's bedchamber to put
the daggers on the guards, which Macbeth forgot to do after he stabbed Duncan to death. 
Lady Macbeth chastises him, telling him that it's no big deal to have blood on his
hands.  She takes the daggers and puts them on the guards, thus getting blood on her
hands.  Both Macbeth and his wife have bloody hands when the knocking begins, so the
timing of the knocking emphasizes the anxiety here as they have to clean up and try to
appear as though they've just been awakened and have no idea a murder was committed in
their house.