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.
register
CC-CEDICT also lists huálā for the figurative sense 'to collapse' (e.g. an organization falling apart). Same characters, different vibe — context tells you which.
