A simile is being used in the story to compare the old mans heartbeat to a watch ticking. This drove the narrator crazy because it gets louder and louder.
There was also a again and again in the story because every night at 12.00 am the narrator would go in his room and watch for his foggy blue eye. He thought the eye was evil so he had to kill him.
The resolution of the story is when the narrator confesses to the cops of what he done. The sound of the old mans heartbeat scared him and he thought the cops heard it too so he screamed out what he did.