HVAC Making Strange Noises? What Each Sound Means

Banging or Booming

What it sounds like: A loud bang or boom when the system starts up or shuts down.

Likely cause: In a furnace, this usually means delayed ignition. Gas builds up before the burners light, causing a small explosion when it finally ignites. This is a safety concern and should be checked by a technician. In ductwork, banging can be caused by expanding and contracting metal when the system cycles on and off. This is annoying but usually not dangerous.

Urgency: Furnace banging = call a technician soon. Duct banging = not urgent but can be addressed during a service visit.

Squealing or Screeching

What it sounds like: A high-pitched squeal when the blower is running.

Likely cause: A worn or slipping blower belt (in older systems with belt-driven motors), dry motor bearings that need lubrication, or a failing blower motor. In AC systems, a squealing outdoor unit often means the compressor is under stress or the fan motor bearings are failing.

Urgency: Schedule service soon. A belt can snap and leave you without heat or cooling. A failing motor will only get worse.

Clicking

What it sounds like: Rapid or repeated clicking, especially at startup.

Likely cause: Some clicking at startup and shutdown is normal. That is the system's relays and contactors engaging. But continuous clicking or clicking without the system starting usually means a faulty igniter, a defective control board, or a bad capacitor. In an AC system, clicking from the outdoor unit without the compressor starting often points to a failing capacitor.

Urgency: If the system is not starting, call for service. If it is just brief startup clicks and the system runs normally, it is fine.

Rattling

What it sounds like: A shaking, vibrating, or loose-part sound.

Likely cause: Loose screws, panels, or mounting brackets on the furnace or air handler. Could also be a loose duct connection. In outdoor AC units, rattling often means loose hardware, a bent fan blade hitting something, or debris inside the unit. In older Fort Collins and Windsor homes, we frequently find loose ductwork connections that rattle during operation.

Urgency: Not usually an emergency, but worth investigating. A loose component can cause damage over time if left unaddressed.

Humming or Buzzing

What it sounds like: A constant electrical hum or buzz.

Likely cause: A gentle hum from the blower motor is normal. A loud hum or buzz from the outdoor AC unit often means the compressor or contactor is struggling. Electrical buzzing can also indicate loose wiring connections, which is a potential fire hazard.

Urgency: Electrical buzzing = call a technician. Normal motor hum = no action needed.

Whistling or Hissing

What it sounds like: Air escaping or a high-pitched whistle.

Likely cause: Air leaks in ductwork, a clogged filter forcing air through small gaps, or undersized ducts creating high air velocity. Hissing near the outdoor AC unit could indicate a refrigerant leak, which needs immediate professional attention.

Urgency: Refrigerant hissing = call immediately. Duct whistling = schedule service when convenient.

Rumbling

What it sounds like: A low rumbling after the furnace burners shut off.

Likely cause: Fuel is still burning in the combustion chamber after the gas valve has closed. This can indicate dirty burners, a problem with the gas valve, or pilot light issues. This is more common in older furnaces we see in Greeley and Loveland.

Urgency: Schedule a service visit. This is not an emergency but should not be ignored long-term.

When to Call Right Away

Call Mountain States Mechanical immediately if you hear:

• A loud bang or boom from your furnace at ignition
• Electrical buzzing or smell of burning
• Hissing from refrigerant lines
• Grinding metal on metal sounds
• Any noise accompanied by the smell of gas

We offer same-day service when scheduling allows across Windsor, Fort Collins, Loveland, Greeley, and all of Northern Colorado. Call us at 970-556-3249 for the earliest available appointment.