While the narrator and Roderick Usher are reading Mad Trist of Sir Launcelot Canning, what is happening in the House of Usher?

When the story is being read, the noises of the story seem to come alive in the home.

At the termination of this sentence I started, and for a moment, paused; for it appeared to me (although I at once concluded that my excited fancy had deceived me)--it appeared to me that, from some very remote portion of the mansion, there came, indistinctly, to my ears, what might have been, in its exact similarity of character, the echo (but a stifled and dull one certainly) of the very cracking and ripping sound which Sir Launcelot had so particularly described. It was, beyond doubt, the coincidence alone which had arrested my attention; for, amid the rattling of the sashes of the casements, and the ordinary commingled noises of the still increasing storm, the sound, in itself, had nothing, surely, which should have interested or disturbed me. I continued the story: