There are several examples of onomatopoeia in the story. As far as a page number, I do not think we have the same texts, but an example would be, ". . . how often was he thrown into complete dismay by some rushing blast, howling among the trees." Remember that an onomatopoeia is a word that sounds like what it is.
A metaphor that appears directly following the above quote is, "All these, however, were mere terrors of the night, phantoms of the mind that walk in darkness."The metaphor is a comparison between two things without the use of "like" or "as". This metaphor is comparing the terrors he discussed previously to "phantoms that walk in darkness".