A modern AC isn't silent, but it should sound the same every day. You get a low hum from the condenser while it runs and a whoosh of air at the registers. There's one click when it starts and another when it shuts off. When one of those changes, the change itself is the clue.
The guide below goes noise by noise, with the likely cause and how urgent it is. Where we publish a price for the fix, it's listed. Everything else gets priced in writing after a $99 diagnostic ($79 if you're new to us), and the diagnostic is waived when you approve the repair.
Hissing or screaming from the indoor unit
Likely cause. Refrigerant escaping from a leak. Rarely, a stuck pressure relief on the compressor.
Urgency. Shut the system off and call now. A hiss from a leak is a warning. A scream is an emergency, because a stuck compressor relief can rupture and cost you the compressor.
Cost. A refrigerant leak search and recharge runs $350 to $650. A leak at a Schrader valve is a small repair. A leak in the coil can mean replacing the coil, which we quote in writing before any work starts.
Banging or clanking from the outdoor unit
Likely cause. Something loose or broken inside the condenser. Often it's a bent fan blade hitting the housing. It can also be a broken compressor mount, or a piston or connecting rod problem inside the compressor.
Urgency. Shut it off and leave it off. A clanging compressor that gets ignored will seize, and that ends the unit's life.
Cost. A failing compressor usually turns into a replace-the-system decision. Compressor replacement runs $1,800 to $3,500, and it only makes sense on an R-410A system under 8 to 10 years old with good warranty status.
Buzzing from the outdoor unit, no fan spinning
Likely cause. A failed start capacitor or contactor. The compressor and fan are trying to start and can't. That buzz is the motor stalled at the start of every cycle.
Urgency. Shut it off at the thermostat. Letting it buzz can damage the compressor windings.
Cost. Capacitor $180 to $320. Contactor $190 to $280. If the fan motor itself has failed, that costs more, and we price it before we start.
Squealing from the indoor unit
Likely cause. A blower motor bearing on its way out. On older belt-drive blowers, which are rare now, it can be a slipping belt. Sometimes something is touching the blower wheel.
Urgency. Soon. A bearing that squeals today can leave you with a dead motor in 2 to 4 weeks.
Cost. Blower motors are priced by model. Variable-speed ECM motors cost the most to replace.
Rattling at startup or shutdown
Likely cause. A loose service panel or duct joint, or metal in the supply trunk expanding as it warms. Usually harmless. At the outdoor unit it can be a loose fan grill, or the equipment pad sinking unevenly.
Urgency. Low. Annoying but not damaging. Tightening panels and resealing duct joints is quick work, and a sinking pad gets shimmed or repoured.
Constant low buzz from registers
Likely cause. Vibration carried through the ducts, often from a blower wheel slightly out of balance. A rattling register damper can do it too.
Urgency. Low. It's a comfort problem more than a mechanical one. Cleaning and balancing the blower wheel fixes it most of the time.
Whistling at registers or returns
Likely cause. Static pressure too high. The system is trying to move more air than the ducts can handle. Undersized returns are one cause. A dirty filter or a blocked supply can do it, and so can an oversized blower.
Urgency. Medium. The system is straining and using more energy than it should.
Cost. A filter swap costs you the filter. Turning down blower speed costs nothing when it's a setting change. Adding a return or resizing ductwork gets quoted after we measure.
Clicking that does not stop
Likely cause. A failing relay or a dirty contactor. A thermostat calling for cooling on and off will also do it. If the clicking comes from the outdoor unit, suspect the contactor first.
Urgency. Medium. A dirty contactor can fail closed, and then the compressor never shuts off.
Cost. Contactor $190 to $280. Relays and thermostats get quoted after the diagnostic.
Gurgling from the indoor unit
Likely cause. Refrigerant flow noise. The system is either low on charge or has air in the lines from a recent service. Sometimes it's just water in the condensate trap.
Urgency. Low to medium. Worth a diagnostic to check the charge.
Cost. If it's low, a leak search and recharge runs $350 to $650. A condensate drain clear runs $150 to $250.
Loud bang on startup, like a gunshot
Likely cause. Liquid refrigerant slugging into the compressor. Overcharge causes it. So do very cold conditions and a failed metering device, and so does a compressor that sat unused for a long time without crankcase heat.
Urgency. High. Slugging fractures the compressor's valves, and the compressor fails.
Cost. A charge correction or metering device replacement gets quoted after the diagnostic. If the compressor is already damaged, you're into the repair-or-replace decision.
Rumble from the gas furnace, especially at startup
Likely cause. Dirty burners, often from rust scale or dust that settled over a summer of sitting idle. A partly blocked flue can do it too.
Urgency. Medium. Rumbling burners often mean incomplete combustion. Have it checked for CO before you run it much more.
Cost. Burner cleaning is part of the $99 heating tune-up. Clearing a flue gets quoted once we see what's in it.
Boom on furnace ignition
Likely cause. Delayed ignition. Gas builds up before it lights, then goes all at once. A dirty igniter or dirty burners will cause it, and so will low gas pressure.
Urgency. High. This can crack the heat exchanger, which is how CO gets into the house. Stop using the furnace until it's inspected.
Cost. Cleaning and a gas pressure check come with the $99 heating tune-up. A new igniter gets quoted on the spot.
The general rule
A noise your system has always made, one that hasn't changed, is almost never a problem. A noise that's new or louder, or just different, is telling you something.
Timing matters most with buzzing from the outdoor unit. That sound is often a capacitor or contactor on its way out, a $180 to $320 repair. Left running for a few days, the same fault can lock up the compressor, and that repair runs $1,800 to $3,500.
That new noise is at its cheapest today. Call 281-992-7866 or book online and we'll put a name on it before it gets expensive.