If you’ve ever spent time around an experienced car mechanic, then you know they understand “car speak.” They can listen to whichever odd noises are emanating from underneath the hood and guess what’s wrong with the engine with starling accuracy.

Cars aren’t the only machines that have their own language. Furnaces also get talkative when something is wrong with them! You can’t talk back to a furnace – at least not in any way that’s productive – but you can learn to interpret the meanings of their many songs. If you’ve ever wondered what your noisy furnace is telling you, then consider this your primer on “Furnacese.”

Please bear in mind that the following information is only intended as a rough guide to furnace noises. Only a residential or commercial HVAC technician is qualified to determine exactly what is wrong with a furnace.


One Pop or Click, Followed by Gentle Humming

This sequence of noises is the furnace’s equivalent of a cat’s contented purring. It simply means that the furnace is turning on and proceeding to produce heat. You may also hear a metallic crackle following the completion of a heat cycle – the normal sound of components contracting as they cool down.



A rattle indicates that two metal components are vibrating against one another – probably because one of them has become loose. A layman may attempt to eliminate rattling by tightening the screws that secure the access panel. If that doesn’t work, then a more sensitive component within the furnace may have loosened. In that case you should rely on a professional to identify and resecure the loose part.



As mentioned earlier, a furnace emits a single clicking sound when it turns on. But if the furnace frequently produces a series of rapid clicks, then its ignition system may be malfunctioning. Alternatively, a broken compressor, control panel or fan motor may be the source of repeated clicking noises. Clicking is an ambiguous furnace sound, which is why we must reiterate that only a trained HVAC pro can diagnose issues with any certainty.



You wouldn’t describe a furnace’s whistle as musical, but it is unmistakable. The high-pitched wheeze is typically produced when a dirty filter forces air to pass through a narrow passage. It’s best not to allow a furnace’s filter to become so dirty that it produces a whistle, as a soiled filter places needless strain on the system’s moving parts. You can avoid a whistling furnace altogether by replacing its filter every three months. Note that whistling can also indicate obstructed air vents or dirty ducts – both simple problems to fix as well.



A malfunctioning oil furnace may produce intermittent knocking sounds because air bubbles have become stuck in its lines. Knocking can also evince a broken ball bearing, belt or gas valve, as well as a dirty or corroded burner. But some percussive furnace noises are innocuous! Several of a furnace’s metal components can make tapping noises while they’re heating or cooling as part of their normal operation.



Furnaces commonly grumble or rumble due to a number of different issues. A throaty growl could result when fuel continues burning in the combustion chamber after the burners have extinguished themselves. Dirt that has accumulated around a burner may make a rumbling noise while it burns up. A pilot light may also declare when it is malfunctioning by grumbling. Like so many issues detailed so far, any of these problems are best left to an HVAC pro.



If you would describe a furnace’s noise as buzzing, humming, purring or murmuring, then you may just be hearing the appliance’s normal operation. An abnormally loud hum may indicate a problem as small as a loose screw or filter, but it can also mean that the blower motor is in immediate need of lubrication or has a failing capacitor. A failing transformer may also create a buzzing noise. These are all serious problems that will eventually lead to total furnace failure if they aren’t addressed quickly.



Air ducts often make banging noises when the furnace blower turns on. Soft banging is common when the metal ducts expand, but loud banging could indicate that the ducts are flimsy or too small. You may be able to stop (or at least soften) duct banging by opening additional vents or replacing your furnace’s air filter. The furnace itself may start banging when its burners are so dirty that they delay ignition. Failing to address that issue may ultimately cause your furnace’s heat exchanger to crack, which could in turn cause a life-threatening carbon monoxide leak.



Many of a furnace’s mechanical components begin to make grinding or scraping noises when they aren’t sufficiently lubricated or are nearing the end of their lifespans. Scraping can also indicate that the blower wheel has come loose from the motor wheel. Reattachment often solves that particular issue, but outright replacement may also be necessary. You increase your chance of paying for the cheaper repair by turning off a grinding furnace and contacting an HVAC technician as soon as possible.



A screeching furnace often contains a malfunctioning belt. The belt may have become stripped, frayed or otherwise worn out, or it may simply have slipped off of its shaft. A blower motor may also emit a high-pitched squealing noise once it has approached the end of its lifespan. In that event, professional replacement is the only practical course of action.

If your furnace has started making any unusual noises, then we welcome you to contact Haugan Heating & Air Conditioning today. We are the greater Sioux Falls, SD area’s resident furnace whisperers, and we’re standing by to take care of all your residential or commercial heating system installation, repair and maintenance needs!