On page 163 of the book, Patrick and Charlie are driving after Brad and Patrick get into a fight. Patrick throws a bottle of wine out of the car and it lands with a crash. In this case, "crash" is an example of a sound word, an onomatopoeia, because that is the sound that glass makes when it breaks.
A good example of a rhetorical question from the book would be a question that Charlie receives from Sam after he royally messes up by kissing her in front of Mary Elizabeth at her boyfriend's house. It is probably the most important rhetorical question in the book, because it encompasses Charlie's insecurities. She turns to him before she runs after Mary Elizabeth and asks, "What the fuck is wrong with you?" (136). Her anger in this case tells us that she doesn't really want an answer.
An example of an idiom in Perks of Being a Wallflower would be on page 107 when Sam tells Charlie that he should "go with the flow" while reading Naked Lunch, because the author was on heroin when he wrote it. "Go with the flow" is a common expression that we use in the United States, but is not found anywhere else, so it is considered an idiom.