Quiet is the whole subject here. The speaker describes a house at night, still and dark, and keeps stripping away every possible noise: no voices, no wind, no cat, not even a trickle or a scatter. The repeated "Not a..." lines work like someone tiptoeing through the rooms, checking each corner and finding nothing.
Then the poem shifts. Instead of sounds, the speaker notices things that can't really be heard: shadows creeping, rustling yawning, the "whisper of falling asleep," the "humble of honest dreams." These are gentle, almost tender images, so the silence feels peaceful rather than frightening. The speaker seems calm, even a little in awe of the stillness.
The mood is hushed and drowsy. The short, broken lines slow the reading down, and the repeated opening line, "House is quiet," returns at the end like a lullaby. The last line, with silence called "invisible," leaves a slightly strange, dreamlike feeling, as though the quiet is so complete it disappears.