Which phrases increase the sense of foreboding in Macbeth?

Throughout the play he does not know whether to believe that the witches' predictions can be falsified. But when he tries to eliminate Fleance and Macduff and fails at both, he begins to believe in the predictions and rely on them as proof that he cannot be defeated. But when he sees Birnam Wood approaching Dunsinane, he begins to fear that the apparently impossible events which are to happen if he is to be defeated are unfortunately not impossible at all.