Answer:
The purpose that the repeated tapping sound at the door serves in Edgar Allan Poe's "the raven" for is that It builds suspense.
Step-by-step explanation:
"The Raven" by Edgar Allan Poe, uses assonance in the first stanza when he introduces the idea of somebody or something tapping at the door, "While I nodded, nearly napping, suddenly there came a tapping, / As of someone gently rapping, rapping at my chamber door" he uses different devices in this lines to generate expectation on the reader.