A modern AC isn't silent, but it should sound consistent. The same low hum from the condenser when it is running, the same airflow whoosh from the registers, the same click on startup, the same click on shutdown. When something changes, that change is the diagnostic.
Below is the field guide. We have organized it by the noise itself, then for each noise we tell you what causes it, how urgent it is, and what a fix typically costs.
Hissing or screaming from the indoor unit
What it usually is: Refrigerant escaping from a leak, or a stuck pressure relief on a compressor (rare).
Urgency: Shut the system off and call now. Hissing refrigerant from a leak is a warning. Screaming is an emergency. A stuck compressor relief can rupture and cost you the compressor.
Cost to fix: Leak diagnosis $150 to $300. Repair varies wildly, $400 for a Schrader valve to $2,500 for a coil leak that requires coil replacement. Refrigerant top-off, $100 to $300 per pound of R-410A depending on supply.
Banging or clanking from the outdoor unit
What it usually is: Loose or broken part inside the condenser. Most often a bent fan blade hitting the housing, or a broken compressor mount, or a piston/connecting rod issue inside the compressor.
Urgency: Shut it off, do not run it. A clanging compressor that gets ignored becomes a seized compressor that ends the unit's life.
Cost to fix: Bent fan blade $120 to $250. Loose mounts $150 to $400. Failing compressor: usually a system replacement decision because compressor replacement runs $2,500 to $4,500 and only makes sense on systems under 8 to 10 years old with R-410A and good warranty status.
Buzzing from the outdoor unit, no fan spinning
What it usually is: Failed start capacitor or contactor. The compressor and fan are trying to start but cannot. The buzzing is the motor stalled at the start of every cycle.
Urgency: Shut off at the thermostat. Letting it buzz can damage the compressor windings.
Cost to fix: Capacitor $190 to $350. Contactor $190 to $350. Both, $300 to $550. Fan motor $400 to $800 if the motor itself is the issue.
Squealing from the indoor unit
What it usually is: Blower motor bearing failing, or a slipping belt on older belt-drive blowers (rare in modern systems), or an obstruction touching the blower wheel.
Urgency: Soon. Failing bearing today, dead motor in 2 to 4 weeks.
Cost to fix: Blower motor replacement, $500 to $1,200. Variable-speed ECM motors on the higher end. Belt $80 to $150 on older systems.
Rattling at startup or shutdown
What it usually is: Loose service panel, loose ductwork joint, or expanding metal in the supply trunk. Often harmless. If it is from the outdoor unit, sometimes a loose fan grill or the equipment pad sinking unevenly.
Urgency: Low. Annoying but not damaging.
Cost to fix: Tightening panels and resealing duct joints, $80 to $200. Equipment pad repour or shimming, $300 to $600.
Constant low buzz from registers
What it usually is: Vibration through the duct system, often from a slightly out-of-balance blower wheel. Can also be a register damper rattling.
Urgency: Low. Comfort issue more than mechanical issue.
Cost to fix: Blower wheel cleaning and balancing $200 to $400. Register replacement $40 to $100.
Whistling at registers or returns
What it usually is: Static pressure too high. The system is trying to move more air than the duct system can handle. Caused by undersized returns, dirty filter, blocked supply, or oversized blower.
Urgency: Medium. The system is straining and using more energy than it should.
Cost to fix: Filter swap, free. Adding a return, $400 to $1,200. Resizing duct, $1,500 to $4,500. Reducing blower speed, no cost if it is a setting change.
Clicking that does not stop
What it usually is: Failing relay, dirty contactor, or a thermostat issue calling for cooling intermittently. If the clicking is in the outdoor unit, the contactor is usually the culprit.
Urgency: Medium. A dirty contactor can fail closed, which means the compressor runs constantly.
Cost to fix: Contactor $190 to $350. Relay $150 to $250. Thermostat $200 to $500 depending on what we install.
Gurgling from the indoor unit
What it usually is: Refrigerant flow noise, often from a system that is either low on charge or has air in the lines from a recent service. Sometimes water in the condensate trap.
Urgency: Low to medium. Worth a diagnostic to see if charge is correct.
Cost to fix: Charge correction, $150 to $400. Drain line clear, $120 to $250.
Loud bang on startup, like a gunshot
What it usually is: Liquid refrigerant slugging into the compressor. Can be caused by overcharge, very cold conditions, a failed metering device, or the compressor sitting unused for an extended period without crankcase heat.
Urgency: High. Liquid slugging will fracture the compressor's valves and lead to failure.
Cost to fix: Diagnosis $150 to $250. Charge correction or metering device replacement $300 to $1,000. If the compressor is already damaged, replacement decision territory.
Rumble from the gas furnace, especially at startup
What it usually is: Dirty burners, often from rust scale or biological dust deposits over a summer of sitting idle. Can also be a partial flue blockage.
Urgency: Medium. Rumbling burners often mean incomplete combustion. Have it inspected for CO before more runtime.
Cost to fix: Burner clean and tune $150 to $300. Flue inspection and clear $150 to $400.
Boom on furnace ignition
What it usually is: Delayed ignition. Gas builds up before ignition, then ignites all at once. Caused by dirty igniter, dirty burners, or low gas pressure.
Urgency: High. This can crack the heat exchanger, which is the CO leak path. Stop using until inspected.
Cost to fix: Tune and clean $200 to $400. New igniter $200 to $350. Gas pressure adjustment included in tune.
The general rule
If your system has always made a noise and the noise hasn't changed, it is almost always not a problem. If the noise is new, louder, or different, it is telling you something. Get a diagnosis before it tells you something more expensive.
"Customer in Alvin heard buzzing from the outdoor unit one Friday afternoon. The unit ran on Saturday and Sunday with the buzzing getting louder. Monday morning the compressor was locked. We replaced it, total cost about $3,200. If they had called Friday when the buzz started, the fix would have been a $250 capacitor. Hearing the change in the noise is the discount window."
Bottom line
You know what your system sounds like. When it changes, treat that as a diagnostic. The cheap fix today is almost always a fraction of the expensive repair next month.
Hearing something new from your AC? Same-day diagnosis. 281-992-7866 or book online.