What does the speaker in "The Raven" intially believe to be causing the noise that he hears?

As the narrator of "The Raven" is sitting in his room, "suddenly there came a tapping,/As of someone gently rapping, rapping at [his] chamber door".  At first he thinks that it is some late visitor at his chamber door, but when he opens the door, noone is there.  He sticks his head into the hallway and whispers the name of his dead lover, Lenore.  All that comes back to him is the echo of his own voice.  When he returns to his chair in his room, he hears a noise again, this time at the window.  He attributes this to "the wind and nothing more".  As the sound continues, he throws back the shutters to find a raven perched outside the window, and it enters the room upon his "throwing back the shutters".