What echoes and footsteps does Lucie hear in chapter 21? (From Book 2 Chapters 21-24)

The echo comes from a corner of Dr. Manette's house,

The corner has been mentioned as a wonderful corner for echoes; it had begun to echo so resoundingly to the tread of coming feet, that it seemed as though the very mention of that weary pacing to and fro had set it going. 64

There is a morbid sense of danger, perhaps the revolution in the sounds. The echo's seem to foreshadow the past catching up with all the characters.

But, there were other echoes, from a distance, that rumbled menacingly in the corner all through this space of time. And it was now, about little Lucie’s sixth birthday, that they began to have an awful sound, as of a great storm in France with a dreadful sea rising. 137