what poetic device is suddenly there came a tapping as of someone gently rapping rapping at my chamber door.

onomatopoeia

The poetic device used in the line you provided is onomatopoeia. Onomatopoeia is a figure of speech where words imitate sounds associated with the objects or actions they refer to. In this case, the word "tapping" imitates the sound of someone gently knocking on the door.

To identify the poetic device in a line or phrase, you can follow these steps:

1. Read the line or phrase carefully and pay attention to any words or phrases that stand out as being descriptive or expressive.
2. Consider if any of the words or phrases imitate sounds, create vivid imagery, or convey strong emotions.
3. Identify any figures of speech that might be present based on your observations. Common poetic devices include simile, metaphor, personification, hyperbole, alliteration, rhyme, etc.
4. Determine which poetic device best fits the description and usage in the given line or phrase.

By analyzing the line "Suddenly there came a tapping as of someone gently rapping, rapping at my chamber door," we can recognize the use of onomatopoeia to evoke the sound of someone knocking gently on the speaker's chamber door.

http://literarydevices.net/internal-rhyme/