A furnace, an air handler and an outdoor unit all make some noise: a click as the thermostat calls, the whir of the inducer and blower, a soft whoosh as the burners light, the steady hum of a compressor. What you want to notice is a sound that is new, louder than before, or tied to one moment (start-up, steady running or shut-down). That moment, and where the sound comes from, say more than the sound alone.
A guide to the sounds
| Sound | Where | Likely cause | How urgent |
|---|---|---|---|
| Loud boom as the burners light | Furnace | Delayed ignition: gas builds up before it lights | Turn off and call |
| Banging or clanking while running | Furnace or air handler | Loose or broken blower wheel, motor mount | Turn off and call |
| Ticking or popping at start and stop | Ducts | Sheet metal expanding and contracting | Usually harmless |
| Squealing | Furnace or air handler | Worn belt on older units, or motor bearings | Book a repair soon |
| Screeching or screaming | Outdoor unit | Compressor in distress, very high internal pressure | Turn off now |
| Rattling | Either | Loose panel or screws, debris, a fan blade touching its guard | Check panels; call if it is inside |
| Humming or buzzing, nothing turns | Outdoor unit or blower | Failed capacitor, seized motor, worn contactor | Turn off and call |
| Repeated clicking, no heat | Furnace | Ignition trouble or a relay or control fault | Call |
| Whistling at registers or returns | Ducts and grilles | Restrictive filter, closed vents, a leaking joint | Check the filter and vents |
| Hissing or bubbling near the lines or coil | AC or heat pump | Refrigerant leak | Call |
| Gurgling | Indoor unit | Partly blocked condensate drain or trap | Call before it overflows |
Treat the table as a starting point. One sound can have more than one source: a rattle may be a loose panel or the first sign of a compressor problem. That is why a technician listens with the panels off and the meters connected.
What you can safely check
- Pin down when. At start-up, while running or at shut-down? In heating, cooling or both?
- Pin down where. Stand by the indoor unit, then the outdoor unit, then a few registers.
- Tighten what’s on the outside. A loose access panel or a buzzing register grille can be snugged up with its own screws.
- Look around the outdoor unit. Sticks, leaves and toys end up near the fan. With the system off, pick up what you can reach from outside the grille; don’t reach inside.
- Check the filter. A clogged or wrong-size filter whistles, and a loose one can be pulled toward the blower.
- Record it. A short phone video with sound helps the technician arrive with the right parts.
Turn it off and call when
- You hear a boom as the burners light.
- The outdoor unit screeches, screams, or hums without starting.
- Metal is striking metal inside the furnace or air handler.
- The noise comes with a burning smell, or a breaker trips.
- You smell gas: leave the house first, then call your gas utility or 911 from outside.
What a technician does
We run the system through a full cycle and listen at each component, then check with the power off: blower wheel and motor mounts, bearings, belts on older units, the outdoor fan blade and motor, the contactor and capacitor, burner ignition, and the refrigerant circuit if there is a hiss. Most noises turn out to be a part that has loosened or begun to wear, which is easier and cheaper to fix before it breaks something else. Repairs use factory certified parts and carry our labor warranty. The work itself is on furnace repair and AC repair; a hiss near the copper lines is covered in refrigerant leaks. If a noise starts at night and you are not sure whether to switch the system off, a real person answers the phone 24/7 and can help you decide; urgent cases go to emergency HVAC.
Decks, brackets and salt air
On steep lots in San Anselmo, Larkspur and Sausalito, outdoor units often sit on decks, platforms or wall brackets instead of a pad on the ground. Those surfaces carry vibration into the house, so a worn rubber isolator, a loose bracket or a fan slightly out of balance can sound far louder indoors than it would on concrete. Near the water, salt air also corrodes fan hardware and fasteners faster than inland, and corroded parts loosen and rattle sooner. A yearly maintenance visit catches a loose part while it is still only a noise.