What is the effect of the knocking at the end of Act 2 Scene 2?

The knocking designates the actual arrival of Macduff and Lennox, but its powerful effect is to startle Macbeth, who has just killed Duncan.  Macbeth's first response to the knocking is: "Whence is that knocking? / How is 'twith me, when every noise appals me?"  That is, "Where is that noise coming from and why does it bother me so much?"  The noise piques his conscience.  The pragmatic Lady Macbeth says they need to get dressed for bed (as if they were awakened by the sound), but Macbeth expresses remorse when he ends the scene with "Wake Duncan with thy knocking! I would  thou couldst!"  The knocking heightens an already suspenseful scene.  At this point, the suspense is so high that Shakespeare introduces comic relief with the porter, who answers the door and jokes about the knocking.  But as he is "the porter of hell gate," we don't forget about the gruesome murder that has just taken place.