轰 (hōng) is itself onomatopoeic - three 车 ('cars/wheels') stacked to evoke a loud rumble - plus 鸣 (míng, 'to cry out / sound'). So 轰鸣 = 'rumble-cry', the deep continuous sound of engines, thunder, or heavy machinery.
usage
Used for low, continuous, powerful sounds - engines, jets, thunder, explosions. For sharp single noises use 轰隆 or 巨响 instead.
