What was scaring the narrator in pit and pendulum?

In the short story "The Pit and the Pendulum" by Edgar Allan Poe, the unnamed narrator is trapped in a chamber during the Spanish Inquisition. He is subjected to various tortures including a swinging blade that slowly lowers above him. What scares the narrator the most is uncertainty - he does not know when the pendulum will strike or even from which side its attack will come, causing him immense psychological torture.