Every heating and cooling system makes some noise: a relay clicks, motors hum, air rushes through the vents, the burners light with a soft whoosh. What deserves attention is a new sound, or a familiar one that got louder. Three questions narrow it down: where it comes from (the furnace or indoor unit, the ducts, or the outdoor unit), when it happens (at start-up, while running, at shut-off), and what it sounds like. A few sounds mean switch the system off now; most mean book a visit soon; some are normal.
Sound by sound
| Sound | Where | What it usually means | What to do |
|---|---|---|---|
| Grinding, metal on metal | Blower or outdoor unit | Failing motor bearings, or a fan blade hitting something | Turn it off, call |
| Bang or boom as burners light | Furnace | Delayed ignition: gas builds up, then lights at once | Turn it off, call today |
| Pop or bang when the blower starts or stops | Ducts | Sheet-metal ducts flexing with the air pressure | Open vents; check if it persists |
| Hissing | Indoor unit or refrigerant lines | Refrigerant leak | Book service |
| Soft hiss at a register or joint | Ducts | Air escaping from a leaky duct | Book a duct check |
| Squealing, screeching | Blower or fan motor | Belt on older blowers, or a bearing starting to fail | Book soon |
| Shriek | Outdoor unit | Compressor under abnormal pressure or failing | Turn it off, call |
| Rattling | Panels, ducts, outdoor unit | Loose panel, screws or duct; debris | Check for loose grilles; book if it stays |
| Clanking | Outdoor unit | Loose fan blade, or something fell inside | Turn it off, call |
| Repeated clicking, no start | Furnace, outdoor unit | Control board, relay or capacitor | Book soon |
| Loud humming | Outdoor or indoor unit | Motor or compressor trying to start | Turn it off if nothing starts |
| Buzzing | Outdoor unit | Contactor, loose parts, debris, electrical fault | Book soon |
| Whistling | Return grille, vents | Restricted airflow | Change filter, open vents |
| Gurgling | Indoor unit | Condensate draining past a partial clog | Check the drain pan |
| Pulsing, rhythmic | Outdoor unit | Often normal with two-stage compressors | Book if it is new |
Turn it off now
Grinding. Metal on metal usually means motor bearings are failing or a fan blade is striking its housing. Running it longer can destroy the motor or the blade. Switch it off at the thermostat and call.
A boom from the furnace. When the burners or igniter are dirty or out of adjustment, gas can collect in the burner area before it lights, then ignite all at once. Repeated small explosions stress the furnace and are a safety concern. This is gas work for a technician only. Make sure your carbon monoxide alarms work. If you smell gas, leave the house and call 911 or the gas company from outside.
Shrieking or clanking outdoors. A high-pitched shriek from the compressor, or the clank of a loose fan blade or something caught in the fan, can turn a repair into a much bigger one within minutes.
Loud humming with nothing starting. A motor or compressor that hums but does not turn is drawing current without moving, and it overheats fast. A weak capacitor is a common cause. If the breaker trips too, read AC tripping the breaker.
Book a visit soon
Hissing from the indoor unit. Often a refrigerant leak. Refrigerant is essential for cooling, and it does not get used up, so a leak has to be found and repaired, not just refilled. That is work for an EPA-certified technician.
Squealing. On older furnaces with a belt-driven blower, a worn belt squeals; on newer ones, a motor bearing starting to go. Not an emergency, but a bearing that squeals today grinds later.
Buzzing from the outdoor unit. Often the contactor, the switch that powers the compressor and fan. It can also be loose parts or debris. A buzz that keeps going when the system is supposed to be off needs checking.
Repeated clicking. One click as the system starts is normal. A string of clicks with nothing starting points to a control board, a relay or a capacitor.
Rattling that stays. Loose panels, screws or a duct strap. If you can see a loose register or return grille, you can tighten it. Equipment panels stay closed.
Usually normal
- A click from the thermostat and the equipment as relays switch on and off.
- A soft whoosh when the burners light and when the blower starts.
- Ticking or light popping from metal ducts as they warm up and cool down.
- A steady low hum from the outdoor unit, and a gentle pulsing from two-stage compressors.
- A heat pump in defrost in cold, damp weather: a whoosh, the outdoor fan pausing, a little steam.
- Water trickling inside the indoor unit while cooling: condensate on its way to the drain.
What you can check yourself
- Whistling: replace a clogged filter and open closed vents.
- Rattling: look for a loose register or grille screw.
- Outdoor clanks or rattles: switch the system off at the thermostat, then clear sticks and leaves from around the unit. Do not reach inside the fan grille.
- Gurgling: look for standing water in the drain pan under an attic or garage unit.
Then note when the sound happens and where, and record a short phone video with sound. It helps the technician find the cause faster.
A note for Hayward homes
In many flatland homes on slab foundations, ducts run through the attic. Hot attic afternoons and cool evenings make metal ducts expand and contract, so ticking around sunset is common and usually harmless. A boom from a furnace that dates from the 1960s or 70s is another matter: dirty burners and worn parts are more likely in original equipment, so call. Loud popping in the ducts can point to closed vents or a duct problem that ductwork repair can fix. For heating sounds book heating repair, for cooling sounds AC repair, and a yearly maintenance visit catches worn bearings and loose parts before they get loud.