young Goodman Brown
This epithet highlights the importance of youth and beauty to American Romanticism. Not only was he referred to " as Young Goodman brown" at the beginning of the story, but in the end too. Throughout the story, this fact about him remains constant.
his heart smote him
Hawthorne personifies Young Goodman Brown's heart and portrays how American Romanticism emphasizes life's passions, one of which is love. This implies that guilt is not part of a thought process, but in fact a response prompted by one's heart. This gives the human heart a more spiritual purpose (rather than biological).
He had been saying to himself—“It is nothing but the wind in the chimney—it is only a mouse crossing the floor,” or “It is merely a cricket which has made a single chirp.”
The use of the word "or", what with its suggestive connotations, paired with the narrator's contemplative tone implies that the old man isn't actually saying these words aloud. Instead, the narrator is inferring what must be going on inside of this man's head; the narrator previously stated that he "knew what the old man felt, and pitied him," which implies a level of empathy and experience.
the mournful influence of the unperceived shadow
This "unperceived shadow" is a metaphor for death; it follows human beings wherever we go, and no-one can get rid of this fate. It's always there, yet we're not always aware of it (like a shadow). The narrator's awareness of (or hyper-fixation on) mortality and ability to identify it in others leads me to believe that his "madness" may be severe anxiety.