HVAC noise is diagnostic. Each sound type — bang, buzz, squeal, rattle, click, hiss — maps to a specific cause or a small set of causes. Use the context (when the sound occurs, which part of the system it comes from, and whether there are other symptoms) to narrow it down.
The one part in this whole chain you fully control:
—
Furnace noises
Loud bang or boom at startup
Cause: delayed ignition. Gas accumulates in the burner box while the igniter warms up. If the igniter is slow to reach ignition temperature, or if the gas valve opens early, gas builds up and ignites with a sharp bang when the igniter finally gets hot enough.
Over time, delayed ignition can crack the heat exchanger — the expanding combustion event puts stress on the metal.
Don’t ignore this. A mild “thump” on startup isn’t unusual, but a loud BANG or boom that you feel as well as hear needs attention. Have a tech inspect the igniter, gas pressure, and burners.
Temporary check: make sure the furnace filter is clean (a clogged filter can cause low airflow that contributes to delayed ignition) and that the gas pressure isn’t too high or too low (a tech check).
Versus: the gentle clicking or popping of metal ductwork expanding — that’s just thermal expansion, occurs throughout the home during heating, and is not a furnace problem.
High-pitched squealing from furnace
Cause: blower motor bearing wear. The blower motor has a bearing or seal that’s drying out. The squeal appears when the blower runs and may stop if you cycle the system off. It will get progressively worse.
A blower motor bearing can eventually seize, which stops airflow entirely.
What to do: if the squeal is consistent and getting louder, have the blower motor serviced or replaced before it fails completely.
Rattling from furnace during operation
Cause: loose panel, blower debris, or rattling duct connection.
- Check that the lower door panel is firmly seated — a loose door panel rattles during operation.
- Debris (a leaf, a screw, insulation) inside the blower compartment can rattle when the blower runs.
- Sheet metal ductwork connections that have come loose rattle as air moves through them.
Turn off the furnace, inspect the blower compartment with a flashlight (no power), and check accessible duct connections.
Clicking from the furnace
Normal clicking: brief clicking 2–3 times at startup (igniter firing, control board relays switching) is normal.
Rapid clicking that doesn’t stop: the igniter is trying repeatedly but not lighting — or the pressure switch is opening and closing rapidly. Check for a flash code and see Furnace Not Heating.
Scraping, grinding metal-on-metal from furnace
Cause: blower wheel contact or failed bearing. The blower wheel has shifted on its shaft, or a bearing has failed enough that the wheel is contacting the housing.
Stop the furnace. Running with a scraping blower can quickly damage the blower wheel beyond repair. Call a tech.
—
AC and outdoor unit noises
Loud hum when outdoor unit tries to start
Cause: failed run capacitor. The compressor or fan motor is trying to start but lacks the phase-shift current to get moving. The motor hums at startup, draws high current, and the unit usually trips the breaker after 20–30 seconds.
Fix: replace the run capacitor. DIY-accessible, $15–40 part.
→ See HVAC Capacitor: Symptoms and Replacement
Buzzing from outdoor unit during operation
Three causes, from most to least common:
- Contactor buzz: the contactor’s electromagnet vibrates when the coil is energized — produces a 60Hz hum or buzz. A worn or chattering contactor buzzes audibly. Replace the contactor if the buzz is pronounced or accompanied by the unit not starting reliably.
- Electrical buzz from loose wiring: a loose electrical connection in the disconnect box or outdoor unit can produce a buzz. Turn off power and inspect visible wire connections for loose terminals.
- Refrigerant leak: a refrigerant leak produces hissing or bubbling that can sometimes sound like buzzing from certain angles. Accompanied by reduced cooling capacity.
Rattling from outdoor unit
Cause: debris, loose screws, or failing fan blade. Leaves, twigs, or other debris can enter the outdoor unit and rattle against the fan blade or coil fins. Also: the top fan guard screws can vibrate loose over time.
Fix: turn off the unit (disconnect and breaker), inspect inside with a flashlight, remove any debris. Check fan guard screws.
A fan blade that’s cracked or has lost a balance weight rattles at low speed and may vibrate at higher speed. Inspect the fan blade visually for cracks or missing material.
Screeching or high-pitched squealing from outdoor unit
Cause: fan motor bearing failure or compressor issue.
- Outdoor fan motor bearing: develops a squeal as the bearing wears. The squeal may come and go as the bearing heats up.
- Compressor: a compressor developing internal problems can make a screeching or whining sound — this is a serious symptom.
A squealing outdoor unit that doesn’t stop after a few minutes needs a tech. If accompanied by other symptoms (poor cooling, high current draw), shut down and call.
Banging or clanking from outdoor unit
Cause: fan blade hitting something, or compressor mechanical failure.
- A fan blade that has slipped or cracked can hit the cabinet at speed — produces a rhythmic banging.
- A failing compressor with internal mechanical damage can bang or clank.
Turn off the system. Running with a fan blade hitting the cabinet will damage both the blade and the coil. Running with a failing compressor can destroy it. Call a tech.
Clicking when AC starts and stops (brief, 1–3 clicks)
Normal. The contactor engaging, the compressor safety valve clicking, or the thermostat relay — brief clicks when the system starts or stops are not a problem.
Rapid clicking that continues: contactor chattering (see buzzing section above), or an electrical fault at the disconnect.
Hissing from refrigerant lines or outdoor unit
Cause: refrigerant leak. A hissing sound from the refrigerant lines, the coil connections, or the outdoor unit (especially from the service valves) indicates refrigerant escaping.
What to do: the system will continue to lose refrigerant until repaired, progressively reducing cooling capacity and eventually damaging the compressor. Call a tech. Refrigerant handling requires EPA 608 certification.
—
Heat pump noises
Heat pump noises include all of the above (it’s similar equipment) plus some that are specific to heat pump operation:
Whoosh and hissing when the heat pump changes modes or defrosts
Normal. The reversing valve switches refrigerant flow direction when the heat pump changes from heating to cooling, or initiates a defrost cycle. This produces a brief whooshing or hissing sound as pressures equalize — lasts a few seconds and completely normal.
Gurgling inside the refrigerant lines
Cause: low refrigerant. When refrigerant level is low, vapor can enter the liquid line, producing a gurgling sound. Accompanied by reduced heating or cooling capacity.
Requires a tech: refrigerant system diagnosis and recharge.
Ice cracking or popping on outdoor unit in winter
Normal in defrost context. When the heat pump initiates defrost and hot refrigerant flows through the iced outdoor coil, the ice cracks and pops as it melts rapidly. This is normal and resolves as defrost completes.
—
Quick reference: sound → cause → urgency
| Sound | Location | Most likely cause | Urgency |
|---|---|---|---|
| BANG/BOOM on startup | Furnace | Delayed ignition | High — can crack HX |
| Clicking (brief, at startup/stop) | Any | Normal relay switching | None |
| Rapid clicking (continuous) | Furnace | Ignition fault | Medium |
| Squealing | Furnace blower | Blower motor bearing | Medium |
| Scraping/grinding | Furnace | Blower wheel contact | High — shut off |
| Rattling | Any | Debris, loose panel, loose screws | Low — inspect |
| Loud hum on startup | Outdoor unit | Failed capacitor | Medium |
| Buzzing (continuous) | Outdoor unit | Contactor, loose wire | Medium |
| Screeching | Outdoor unit | Fan motor bearing or compressor | High |
| Banging/clanking | Outdoor unit | Fan blade contact or compressor failure | High — shut off |
| Hissing | Refrigerant lines | Refrigerant leak | Medium — call tech |
| Whoosh/hiss (brief, mode change) | Heat pump | Normal reversing valve | None |
| Gurgling inside lines | Heat pump | Low refrigerant | Medium |
—
FAQ
My furnace makes a ticking sound throughout the house when heat is running. Is that a problem? Almost certainly not — that’s ductwork expanding. Sheet metal ducts tick and pop as warm air heats the metal. It’s most noticeable on the first heat call of the day. It’s not a furnace issue; it’s a structural characteristic of sheet metal ductwork.
My AC makes a brief sound every 5 minutes when I can’t even hear the outdoor unit. What is that? Likely the thermostat relay clicking. Some thermostats click audibly when switching the cooling call on or off. This is normal and no different from a light switch clicking.
The outdoor fan on my AC wobbles when it spins. Is that bad? A slight wobble is normal due to blade balance. A noticeable wobble, or one that’s getting worse, indicates a blade that’s bent, cracked, or has lost its balance weight. Shut the unit off and inspect the fan blade — a significantly damaged blade should be replaced before it breaks and damages the coil.
