Normal and Not-Normal Sounds from Your Heating and Cooling

Why this matters

Your furnace, air conditioner or heat pump makes a sequence of sounds every time it starts, and most of them are the system doing exactly what it should. A few sounds mean a part is wearing out, and a very few mean turn it off now. Knowing which is which saves you a worried night over a normal click, and it keeps you from running a system that is grinding itself apart. A short video of the sound is also the single most useful thing you can hand the tech who comes out.

First, the sounds that are not a "wait and see": a loud bang or boom when the burners light, any sound that comes with a burning or electrical smell, smoke, or a sounding carbon monoxide alarm. For a bang, a burning smell or smoke, turn the system off at the thermostat; for smoke you cannot account for, get out and call 911. For a sounding CO alarm, get everyone outside first and follow How to Respond When Your Carbon Monoxide Alarm Goes Off. See When to Stop and Call a Licensed Pro Around the House for the full list. If you smell gas, do not touch the thermostat or any switch: leave the house and call the gas utility from outside.

What a normal heating cycle sounds like, start to finish

A gas furnace runs the same short script every time. Learning it once lets you hear where a problem starts.

1. A soft click at the thermostat or furnace. That is a relay closing to call for heat.
2. A hum or whir from the furnace cabinet. On most modern furnaces this is the inducer motor, a small fan that pulls combustion gases through the furnace and up the vent before the burners are allowed to light. It runs for several seconds first.
3. A tick-tick-tick or a faint glow-and-pause. That is the ignition: either a spark igniter clicking or a hot-surface igniter heating up (that one is usually silent).
4. A soft whoosh. The gas burners light. It should be a smooth, quick sound, like a gas stove burner catching.
5. A pause, then the main blower comes on. Most furnaces wait until the heat exchanger is warm before blowing air through the house, commonly somewhere around half a minute to a minute and a half. That delay is deliberate so you do not get a blast of cold air.
6. Ticking or light popping in the ducts. Sheet-metal ducts expand as warm air hits them, and they tick as they warm and again as they cool. It can be loud in a bedroom where a duct runs across a joist.
7. At the end: the burners shut off quietly, the blower keeps running for a minute or two to pull the last heat out, then everything stops.

An electric furnace or air handler skips the burner steps: you may hear a click as heating elements switch on, a faint hum, then the blower.

Cooling and heat pump sounds that are normal

• The outdoor unit starting. A click (the contactor, an electrical switch in the outdoor unit), then the hum of the compressor and the whoosh of the fan on top. A short shudder at startup and shutdown is common.
• Airflow at the vents. A steady hush of air is normal. A whistle at one register usually means that register is partly closed or a filter is loaded.
• The heat pump defrost cycle. In cold, damp weather the outdoor coil grows frost. Every so often (commonly somewhere between every half hour and every hour and a half when it is frosty) the heat pump reverses itself to melt it. You hear a loud whoosh as a valve shifts, the outdoor fan stops, and you may see a cloud of steam rising off the unit. After a few minutes, usually under about ten, it whooshes again and returns to heating. This alarms a lot of first-year heat pump owners and it is normal. See related: Your First Winter with a New Heat Pump.
• Gurgling or dripping near the indoor unit. Cooling and high-efficiency furnaces produce water that drains away through a small pipe. A gentle gurgle as it drains is normal. Water on the floor is not; see How to Clear Your Air Conditioner Condensate Line.

The sounds that earn a call

These do not all mean the same urgency. The table says what to do while you wait.

A system you turned off for any row above stays off until the tech has found the cause. Turning it back on is the tech's step, and it is back only after a full cycle runs start to finish with no repeat of the sound.

Why the outdoor unit stays closed. It contains capacitors that hold an electrical charge even after the power is off, and the fan can start without warning. The disconnect box beside it is there for the tech. Turning the system off at the thermostat is enough to stop a buzzing unit from hurting itself.

The repeated-click case, worked through

What happened. It is a January evening. The thermostat is set to 68 and reads 63, and the house is getting cooler. From the basement you hear the furnace hum, then a clicking, then nothing. A few minutes later it happens again. Then it goes quiet and stays quiet.

What to do first. Nothing in this story involves a gas smell, a bang or an alarm, so this is a "record it, then call" problem, not an emergency. Check the two things in your safe set: that the furnace switch (it looks like a light switch on or near the furnace) is on, and that the filter is not packed solid. Both were fine here.

What is going on. Most furnaces try to light a few times, often three, and then lock themselves out for a while, frequently about an hour, before trying again. The exact count and wait time belong to the manufacturer. The lockout is a safety feature: the furnace will not keep dumping gas into a burner that will not stay lit. That is why the most common homeowner mistake is the wrong one. People turn the switch off and on to "reset" it four or five times, and each reset starts a fresh round of tries. A tech wants to see what the furnace does on its own, and repeated resets can also leave unburned gas in the burner area, which is exactly what produces the bang in the table above.

What to record. Instead of resetting, wait for the next try and record it. The video below was taken by standing a phone where it could see the furnace's small viewing window or burner area without removing any panel, and narrating the time out loud.

SOUND LOG - furnace, basement
thermostat: set 68, reads 63, outside 19 F

0:00  click at furnace, inducer hum starts
0:22  orange glow at igniter (seen through window)
0:39  whoosh, burners light, blue flame
0:43  flame goes out, burners off, inducer still on
1:10  second try: glow, whoosh, flame out at 1:15
1:48  third try: glow, whoosh, flame out at 1:52
2:05  inducer stops, all quiet
      (no restart after 25 more minutes of waiting)

Why this log is worth so much. It tells the tech the furnace got through its first steps (the inducer ran, the igniter glowed, gas flowed and lit) and failed at one specific point: the flame lit and then went out after about four seconds, three times in a row, and then the furnace locked out. That narrows the visit before the tech reaches your door. Compare that with "it clicks and doesn't work", which could mean almost anything.

What would change the picture. If you had heard clicking with no glow and no whoosh at all, the failure is earlier in the sequence. If the burners lit with a loud bang, the instructions change completely: turn it off at the thermostat and do not try again. If the house is dropping toward freezing and the tech cannot come soon, keep faucets on outside walls dripping and see Frozen Pipe Prevention + Thaw Reference. Never use an oven or a gas range to heat the house.

How to record a sound so it helps

A tech can do far more with a good recording than with a description.

• Start before the sound. Begin recording as the system is called on at the thermostat, so the whole start sequence is captured. A clip of only the bad noise loses the context of when it happens.
• Say the time and conditions out loud. Thermostat setting, the reading, the outdoor temperature, and whether it is heating or cooling.
• Show which unit. Pan once from the unit to its label so the tech knows the type (see Reading the Labels on Your Furnace and Air Conditioner).
• Keep the phone still for a full minute or two. Short clips miss intermittent sounds.
• Stay outside the cabinet. Record through the viewing window or from beside the unit. Do not remove panels, and stay out of the outdoor unit entirely.
• Write down the pattern. Every cycle, only first thing in the morning, only when it is below freezing outside. That pattern often matters more than the sound itself.

Sounds that seem bad and usually are not

• A loud thunk when the outdoor unit stops. Compressors often shudder on shutdown. If it is new and much louder than before, mention it at the next visit.
• Water rushing in walls when cooling. Usually the condensate drain, or refrigerant moving through the lines. A steady hiss that does not stop is different.
• Popping in the ducts at night. Thermal expansion again, and louder when the house is quiet.
• A brief burning-dust smell with the first heat of the fall. Dust burns off the heat exchanger for the first run of the season. See Smells in the House and What Each One Might Mean for when it is not dust.

Where your part ends

Your part: listen, record, change the filter, change the thermostat battery, make sure the furnace switch is on, keep the outdoor unit clear of leaves and snow, and turn the system off at the thermostat when it is grinding, banging or buzzing. The licensed tech's part: opening the cabinets, testing electrical parts, anything to do with gas, refrigerant or the burners. If a sound returns after you have done your part, the next step is a service call with the video in hand. See related: What to Expect During a Service Visit.

References

• See related: When to Stop and Call a Licensed Pro Around the House
• See related: What Your Carbon Monoxide Alarm Is Telling You
• See related: Your First Winter with a New Heat Pump
• See related: Frozen Pipe Prevention + Thaw Reference
• Furnace and heat pump installation and owner's manuals for the ignition retry count, lockout timing and defrost behavior of your specific unit