Answer: He thinks it is a visitor.
In the first stanza of the poem "The Raven," the speaker hears a tapping at his chamber door around midnight. Wondering who could be at that hour, he assumes it is a visitor. The stanza says:
Once upon a midnight dreary, while I pondered, weak and weary,
Over many a quaint and curious volume of forgotten lore—
While I nodded, nearly napping, suddenly there came a tapping,
As of some one gently rapping, rapping at my chamber door.
“’Tis some visitor,” I muttered, “tapping at my chamber door—
Only this and nothing more.”