Why Is My HVAC Making a Strange Noise?

A banging or clanking sound from the indoor or outdoor unit is one of the more urgent noises — it typically means a loose or broken component is moving around inside the equipment. In the outdoor unit (condenser), this is often a loose fan blade that is striking the housing or a broken mount allowing the compressor to move. In the indoor unit (air handler or furnace), it can be a loose blower wheel, a broken motor mount, or debris that has entered the unit.

If you hear a banging or clanking sound, turn the system off at the thermostat immediately. Running the system with a loose or broken component can cause additional damage quickly — a fan blade that is striking the housing will eventually damage the housing itself, and a loose blower wheel can damage the motor. Call a technician and describe the sound and which unit it is coming from. This is not a noise to monitor and wait on.

A high-pitched squealing or screeching sound usually comes from the blower motor. In older systems with belt-driven blowers, this is often a worn or slipping belt — the same sound as a slipping fan belt in a car. Belt replacement is a straightforward repair. In modern systems with direct-drive motors (no belt), squealing typically indicates a failing motor bearing. Bearings that are dry or worn will squeal before they fail completely.

A squealing blower motor bearing will eventually seize, at which point the motor fails entirely and the system stops moving air. Addressing it when the squeal starts — before failure — is significantly less expensive than an emergency motor replacement. If the squeal is coming from the outdoor unit, it may be the condenser fan motor bearing rather than the blower. Either way, schedule a service call promptly.

A single click when the system starts or stops is normal — it is the sound of the contactor (an electrical relay) engaging or releasing. What is not normal is clicking that continues during operation, rapid clicking that repeats without the system starting, or clicking that occurs when the system should not be running at all.

Continuous clicking during operation often indicates a failing relay or control board. Rapid clicking without startup is a common symptom of a failing capacitor — the capacitor provides the starting torque for the compressor and fan motors, and when it is weak, the motors attempt to start, fail, click, and try again. A failing capacitor is one of the more common AC repairs and is relatively inexpensive to replace. If the clicking is accompanied by the system not starting, call a technician.

Rattling is one of the more varied noises — it can come from several different sources. The most benign cause is a loose access panel on the furnace or air handler. Check that all panels are fully seated and secured. Rattling from the ductwork is also common and usually harmless — sheet metal ducts expand and contract with temperature changes, and loose duct connections can rattle when air is flowing.

Rattling from inside the unit is more concerning. Debris (leaves, twigs, small animals) can enter the outdoor unit through the top grille and rattle around when the fan runs. Loose components inside the unit — a loose heat exchanger panel, a loose motor mount, or loose screws on the blower housing — can also rattle. If the rattling is coming from inside the equipment rather than the ductwork or panels, have a technician inspect it.

A low hum from the outdoor unit during operation is normal — it is the sound of the compressor and fan motor running. A louder buzz or hum that is new, or that occurs when the system is not running, typically indicates an electrical issue. Common causes include a failing capacitor (which produces a hum as it struggles to maintain charge), a loose electrical connection that is arcing, a failing contactor, or a compressor that is trying to start but cannot.

A buzzing sound from the indoor unit can be a loose transformer, a failing relay, or electrical arcing at a connection. Electrical issues should be addressed promptly — arcing connections can cause component damage or, in rare cases, fire. If the buzzing is accompanied by a burning smell, turn the system off immediately and call a technician.

Banging or popping sounds from the ductwork — rather than from the equipment itself — are usually caused by the sheet metal expanding and contracting as it heats and cools. This is most common in the first few minutes after the system starts and is generally harmless. It is more pronounced in systems with thin-gauge ductwork or with duct runs that are long and straight.

If the banging is loud, frequent, or occurs throughout the run cycle rather than just at startup, it may indicate undersized ductwork. When ductwork is too small for the system's airflow, the negative pressure on the return side can cause the duct walls to flex inward with each blower cycle — a condition called oil-canning. This is not dangerous, but it indicates an airflow restriction that is reducing system efficiency and potentially stressing the blower motor. A technician can assess whether duct modifications would help.