Onomatopoeia for any rushing/crashing sound - pouring rain, collapsing walls, rolling coins, gushing water. The reduplicated 哗啦哗啦 is more vivid; 哗啦一声 'with a crash' marks a single sudden noise.
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There is also a figurative reading huálā for 'to collapse' (e.g. an organization falling apart). Same characters, different vibe - context tells you which.
