Hmmm. I found one example of what might be considered onomatopeia in the phrase "A snowstorm of cotton-grass seeds blew past her face." As critic M.H. Abram notes, "there is no exact duplication of non-verbal by verbal sounds; the percieved similiarity is due as much to the meaning, and to the feel of articulating the words, as to their sounds."
Alexander Pope says that "the sound should seem an echo of the sense."
As the wilderness in this novel is as much a character as Julie herself, you can surely find many other more approriate examples of language that mimics the senses.
A Glossary of Literary Terms, 7th ed.
Ft. Worth: Harcourt Brace, 1999
The wilderness symbolizes escape for Julie. It represents pleasure, virtue, and love. The city life would then represent stress and all of the other headaches that go with the civilized world.
I don't have a specific example of onomatopoeia, but I can perhaps lead you in the direction to find one. Onomatopoeia is more than just words that imitate sounds, such as "buzz". Writers also choose words whose sounds suggest their denotative (dictionary meaning) and connotative (the emotions a word evokes)meanings. Some examples would be whisper, kick, gargle, and clatter. In the book, look for perhaps the sounds Julie hears in the wilderness or sounds made by the animals there.