What causes whistling or hissing sounds from my Toronto home ductwork?

Whistling or hissing sounds from your ductwork are caused by air being forced through a restriction or gap at high velocity, and in Toronto homes the most common culprits are undersized ducts, closed or partially blocked registers, dirty filters, leaking joints, and flex duct that has become kinked or crushed over time. The sound is essentially the same physics as whistling through pursed lips — air accelerating through a narrow opening creates turbulence and noise.

The most frequent cause in GTA homes is a dirty or restrictive furnace filter. When a filter becomes clogged with dust, pet hair, and debris — which happens quickly in Toronto's pollen-heavy spring and dusty summer construction season — the blower motor pulls air through the restricted filter media at high velocity, creating a hissing or whooshing sound at the return grille. Check your filter first. If it is visibly dirty, grey, or clogged, replace it. Filters should be changed every one to three months depending on type, and more frequently if you have pets or nearby construction. A clean filter alone may resolve the noise completely.

Undersized ductwork is a systemic issue common in post-war Toronto homes across Scarborough, North York, and Etobicoke. These homes were built with ductwork designed for heating only, and when air conditioning was added later, the existing ducts were too small to handle the higher airflow requirements (cooling requires approximately 400 CFM per ton versus 300 CFM per ton for heating). Air moving through undersized ducts at excessive velocity creates whistling at registers, rushing sounds through trunk lines, and turbulence noise at fittings. If your ductwork has always been noisy since the air conditioner was installed, undersized ducts are the likely cause, and the only real fix is upsizing critical trunk lines and branch runs — a project that typically costs $3,000 to $8,000 depending on scope.

Partially closed registers and dampers are another common source. When a register is nearly closed, the small opening accelerates air to high velocity, creating a distinct whistle. Check all your supply registers — open them fully and see if the noise stops. If someone previously closed registers in unused rooms to redirect airflow, this increases static pressure throughout the system and can cause whistling at remaining open registers. The proper solution is to keep all registers open and use zone dampers in the trunk duct if you need room-by-room control.

Flex duct problems are widespread in GTA homes built from the 1980s onward. Flex duct that has sagged from its hangers, kinked at a turn, or been compressed by stored items in the attic creates a restriction that whistles or hisses as air pushes through. In attic spaces, flex duct insulation sometimes separates and bunches up inside the outer jacket, creating an internal blockage. Inspecting attic flex duct runs and re-supporting sagged sections, straightening kinks, and pulling the duct taut can eliminate noise from these sources.

Leaking joints produce a hissing sound as pressurized air escapes through gaps. Run your hand along accessible duct joints while the system is running — you will feel air blowing from leaky connections. Seal these with duct mastic for a permanent fix.

If the noise started suddenly rather than gradually, check whether a damper inside a duct has come loose and is partially blocking the airflow path, or whether a register boot has separated from the branch duct. These issues are common after renovation work or heavy foot traffic above floor registers. If you have addressed the obvious causes and the whistling persists, a ductwork contractor can perform a static pressure test to identify the restriction point — this diagnostic test typically costs $150 to $300 and pinpoints exactly where the problem lies.