Noises in the Walls at Night: Mouse, Rat, Squirrel, or Something Else?
A noise inside a partition rarely wakes anyone at the right moment to analyze it. Yet three pieces of information you already have — the time, the position, and the type of sound — identify the species in the great majority of cases. And the species determines everything else: urgency, method, cost, and structural risk.
The time of the noise: the first sort
A noise that starts at dusk and continues through much of the night points to a rodent — mouse or rat. These animals are strictly nocturnal and their activity peaks in the two to three hours after sunset, then resumes before dawn.
A noise that appears at dawn and in late afternoon, with complete silence at night, almost always indicates a squirrel. It is a diurnal animal, and that distinction alone eliminates half the hypotheses. A noise occurring in the middle of a summer day, in a south-facing wall, points instead to a wasp nest or a carpenter ant colony.
Height in the wall: where the animal travels
Mice travel at every level but favour interior partitions and the voids between floor joists. Scratching that seems to come from the floor or the base of a wall, particularly near the kitchen, matches the classic house mouse profile.
Rats prefer the bottom of the building: basement, crawl space, base of exterior walls. Squirrels occupy the top — attics, soffits, the space under the roof — and the noise moves horizontally above the ceiling rather than vertically inside a partition. That difference in trajectory often tells you more than the sound itself.
The type of sound: scratching, running, or rolling
Light, rapid scratching, like a fingernail on cardboard, with frequent pauses, matches a mouse. The sound is faint and you often have to strain to hear it. A heavier running noise, with perceptible weight and slower, more deliberate movement, indicates a rat.
A rolling sound — the impression of an object moving above the ceiling — signals a squirrel carrying nuts or nesting material. A continuous, diffuse rustling with no identifiable movement, inside a warm wall, suggests insect activity instead: wasps in summer, carpenter ants in spring.
Why identification changes the intervention entirely
A mouse enters through a six-millimetre opening and is handled with baiting and targeted sealing. A rat requires a two-centimetre opening and causes markedly greater structural damage, notably to electrical wiring — a documented fire risk factor. The method, the equipment size, and the budget all differ.
A squirrel follows a different logic again: it is exclusion, not extermination. You install a one-way door, wait for the animal to leave, then seal permanently. Treating a squirrel like a pest rodent regularly leads to a dead animal inside an inaccessible wall, with the odour consequences you would expect.
What not to do right away
Do not open the wall. It is the first instinct and the worst: you lose containment, disperse the animal into the rest of the building, and turn a targeted intervention into an open hunt. The wall is opened only after identification, and only if access is indispensable.
Do not put out poison blindly either. A poisoned rodent dies where it happens to be, often inside a closed cavity, and the odour lasts three to six weeks with no way to reach it. Baiting goes at identified travel points, after inspection, and never inside a wall void.
When to call without waiting
Three situations justify an immediate call rather than a period of observation: a noise in a wall carrying electrical wiring, an ammonia smell accompanying the noise — a sign of accumulated urine and therefore an established population — and the presence of children or immunocompromised people in the home.
In other cases, note the time, position, and nature of the noise over two or three nights. Those notes are worth more than a long phone description and often let the technician arrive with the right equipment on the first visit.
Hearing something and unsure of the species? The ParasiteMontréal inspection is free: the technician identifies the animal, locates the entry point, and tells you what is urgent and what can wait.
