The Clennam house is creepy and gloomy and never sees sunlight or moonlight. It's also a beat-up wreck.
One night Affery has another "dream" (or at least another experience like the one Flintwinch told her was a dream.)
She "dreams" the she overhears Flintwinch and Mrs. Clennam arguing.
Flintwinch tells Mrs. Clennam never to yell at him, and Mrs. Clennam is angry that he implied too much to Arthur when Arthur visited.
Flintwinch says that even though he was never a huge fan of Mr. Clennam, it's not OK to slander the dead, so he needed to make sure Arthur knew that Mr. Clennam was blameless.
(Blameless of what, we all wonder, but we don't yet know.)
Affery is floored that Flintwinch is talking so forcefully to Mrs. Clennam and that she's just taking it. Since he is her servant, and since she's a human lead pipe, she usually does the bossing.
The conversation is over, and Affery runs back into the kitchen to hide from Flintwinch. She hears a scary noise from somewhere within the walls of the house.
When Flintwinch finds her, she is terrified, and his explanation of the noise as "rats, cats, water, drains" (1.15.70) doesn't make her feel any better.
From then on, Affery spends her days wondering what on earth Flintwinch and Mrs. Clennam are up to, being scared of the crazy noises she keeps hearing, and trying to attract as little attention as possible.