In the middle of the night, we suddenly heard the rat-tat-tat of someone banging on the door violently.

rat-tat-tat 是拟声词，模拟连续急促的敲击声，常用来形容敲门声、机枪声等有节奏的敲打声。这个词通过重复音节"tat"来强调声音的连续性和急促感。