* They are words: Unlike sound effects, they are part of the language.
* They don't require special emphasis: Their meaning is clear from their sound.
Examples:
* The car whizzed past.
* The dog barked loudly.
* The rain pattered on the roof.
Exceptions:
* Emphasis: You might use quotation marks if you want to emphasize the sound, but it's not common.
* Dialogue: If the onomatopoeia is part of a character's dialogue, you would use quotation marks as usual.
Remember: Use your judgment and consider the context to decide if quotation marks are necessary.