Traditionally: "Crash" is considered an onomatopoeia. It mimics the sound of something breaking or colliding.
However: Some linguists argue that "crash" is actually a metaphorical onomatopoeia. This is because the sound of "crash" isn't a perfect representation of every kind of crash. Think about the difference between a car crash, a glass breaking, and a thunderclap. Each of these might be described as "crashing," but the actual sounds are distinct.
Therefore: While "crash" is often perceived as an onomatopoeia, its relationship to sound is more complex than a perfect representation. It acts as a general term for a loud, sudden, and destructive noise.