Reviewed by George Baral
Updated March 14, 2023
Bangs and booms are great when you’re listening to fireworks, but you probably don’t want to hear percussive noises coming from your home’s HVAC system. Such noises are usually signs of trouble with a furnace or air conditioner.
What do those sounds mean and when do you need to take action?
HVAC stands for heating, ventilation, and air-conditioning. HVAC systems include heating and cooling units, blowers, air ducts, dampers, return grills, vents and the air filter that provide heating and cooling to your home.
In most homes, the HVAC system is quite extensive. When it starts to act up and make odd noises, it can be hard to figure out where the noises are coming from and what’s causing them.
“The first thing homeowners should do when they hear new noises coming from their HVAC is to turn off their system and read through the manual guide,” says Glenn Wiseman, Top Hat Home Comfort. “Sometimes noises have easy fixes, and the reason behind the noise is written in the manual. You can always call the company or manufacturer that installed your system; they might provide insight into the cause of the noise.”
Marla Mock, VP of Operations for Aire Serv, agrees that noises often signify problems:
“While it’s normal for an AC unit to make a soft humming noise, majority of the times noises aren’t normal and homeowners should make a point to address it as soon as possible.”
Here are some common HVAC noises and what they mean:
It’s normal for your HVAC system to hum as it works, but when you begin hearing unusual noises or the hum gets noticeably louder, it’s time to investigate. The system spans your entire home, so it may be difficult to pinpoint the source of the noise. Check the various system components in turn.
Sometimes, an HVAC noise just indicates that the unit needs maintenance, but it can also signal the need for the replacement or repair of parts. Don’t ignore such noises; even if there’s only a minor problem, leaving it to fester could mean more expensive damage in the future. It’s best to call a professional for evaluation.
A home’s ductwork system is generally made of sheet metal in a network of pipes, dampers — valves or plates that regulate air flow — and sealants. You’ll often hear air exiting vents into your rooms but be alert to less ordinary sounds.
Popping and banging, for example, result when the temperature changes and duct walls expand and contract in response.
Rattling can occur when loose metal pipes knock against each other. As your system ages, the sealant isn’t as effective, so vibration and static pressure can cause ducts to come loose. If you have a clogged filter that is blocking airflow, there can be a pressure drop that causes shaking and vibrating ducts.
Booming sounds are quite common at the spot where your ductwork connects to the plenum — the air distribution box that facilitates air circulation. It’s the spot where the largest temperature swing occurs and there may not be enough dampers or expansion joints to isolate the sound.
Buzzing or scratching in your ducts may be caused by mice, bees, raccoons or squirrels that have somehow entered your ductwork.
“[Frequent clicking] often means there is loose hardware inside the compressor,” says Mock. “To identify the issue, take a look at the compressor and see if there are any loose springs or bolts that can be safely removed or tightened to eliminate the clicking and ticking noise.”
“If this sound is brief and doesn’t last beyond startup, then there is nothing to be worried about,” says Mock. “However, if your unit never used to squeal but recently started doing so, you may need to repair your unit. Hire a licensed and professional HVAC tech to inspect the unit for any possible issues.”
Hissing sounds from an HVAC system are rarely good.
“If you hear [hissing], then call a professional right away,” says Mock. “Hissing sounds indicate a catastrophic refrigerant leak that is being dumped into the atmosphere. You’ll need a refrigerant line replacement and recharge from a professional to help get your AC unit up and running again.”
On bubbling sounds from HVAC systems, Mock says: “a bubbling noise stems from a slow leak that introduces gas bubbles into the refrigerant line. If you hear this sound, call a heating and air conditioning specialist to perform a leak detection and repair as soon as possible to avoid further damage.”
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Unlike many duct sounds, you shouldn’t take unexpected noise from your furnace in stride. Most sounds are an indication that the furnace needs attention, and not addressing the issue could be hazardous — furnaces can leak deadly carbon monoxide into your home.
Knocking is generally a benign noise; it usually indicates that your vibration damper isn’t installed correctly, so sounds from the blower reverberate throughout the system. Rumbling, rattling or huffing and puffing sounds are more worrisome.
“Furnace rumbling is a sign of an issue with the burner,” says Wiseman. “It needs to be given immediate attention.”
Rattling sounds in your furnace can also signify a cracked heat exchanger — one of the most common causes of carbon monoxide leaks. If you suspect a cracked heat exchanger, do not use the furnace until it has been replaced.
Squealing can often be attributed to the heat pump or the blower. Squealing may mean that there’s a belt that’s frayed or that the blower and its housing need a cleaning.
“Whistling happens when something impedes proper airflow caused by a dirty air filter or a closed vents or dampers in the ductwork,” says Wiseman. “Usually, if you change the filter, you might be able to solve the noise; however, you will need to get a technician to fix the furnace in some cases.”
Grinding or scraping are generally serious. It often means the blower motor is failing, due to improper maintenance or the end of its lifespan.
“Furnace banging can signal an issue with the ignition, which means that there is gas build up in the combustion chamber or due to contraction and expansion of the ductwork,” says Wiseman. “Problems with the ignition need to be assessed by a professional.”
Today’s air-conditioners are quieter than ever. They are equipped with sound-dampening technology and two-speed compressors that keep the noise level under 55 decibels. If you hear odd sounds coming from your air-conditioning system, pay attention. They could indicate the need for a tune-up, but they could also be the harbinger of a more serious problem.
“Noises coming from the air conditioner can take many forms,” says Albert Lee, Founder of Home Living Lab. “In the majority of the cases, air conditioner noises are due to a loose component in the unit such as a loosening compressor, bolts or even a twig that may have gotten into the unit by accident. The noises generated by these loose components are generally lower pitched and may produce sounds like buzzing, humming, banging or clanging.”
If you hear banging, it’s generally a sign that there’s a loose part in the compressor, while clanking signals that there’s a part that’s loose or out of balance. You shouldn’t be surprised to hear clicks when your air-conditioning starts or shuts down, but regular clicking suggests that there may be a failing thermostat or a defective control. A humming sound usually isn’t serious — perhaps a loose part — but if you don’t investigate, it could lead to more serious damage.
Squealing indicates a potential issue with the blower motor or the fan motor. Buzzing denotes a variety of problems, such as unbalanced fan blades, an outdoor motor about to fail, a condenser coil that needs cleaning or refrigerant leaks.
Chattering or rattling are unwelcome noises, because they can mean that your unit is beginning to deteriorate; of course, they can also suggest that there are twigs clogging the system.
The most serious sound that your air conditioner can make is screaming. When that occurs, it’s a signal to shut down the system immediately and call an HVAC professional.
“If you hear a loud high pitched whistling or screaming sound from the air conditioner, it is very likely a refrigerant leak which is highly damaging to the unit and may also have harmful effects on our health,” says Lee. “Loud screaming may also be due to high internal pressure in the air conditioner which may be dangerous. Turn off the unit immediately when you hear these noises and call a professional immediately.”
With so many different noises, and not all of them problematic, it’s hard to be sure when you need professional intervention.
“If you begin to suddenly hear new noises from your AC unit, then you should immediately seek out an expert,” says Brandi Andrews, Founder & CEO at National Air Warehouse. “Air conditioners are delicate and there are tons of things that can go wrong extremely quickly for no real reason. Maybe some dust collected on the fan, or maybe a critter got into it — it doesn’t take much!
“Call an expert to come to take a look. Don’t commit to getting any work done and try to find someone who will take a look for no charge. Only someone who has trained to work with air conditioners and is an expert will know what to do to make sure your system is operating at its highest level.”
If you’re not sure if a noise is cause for concern, it’s best to be on the safe side when it comes to contacting a serviceperson.
“Homeowners need to consider what noises cause concern and need to be assessed by an HVAC technician,” says Wiseman. “It is always best to call a professional to come to evaluate the problem and provide a recommendation for the next steps. Even if the problem is minor, it is still best to get expert knowledge from a technician to protect all of those in your home.”
If your HVAC system is making unusual noises, it’s trying to tell you something, so don’t ignore it. HVAC systems are costly, and you want them to last as long as possible, so take good care of them.
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About the expert: George Baral
George Baral has an MBA and a masters' degree in chemistry. He spent almost 35 years inspecting and evaluating heating and air conditioning systems before retiring. He obtained a California general contractor's license to start a company focusing on energy-efficient construction, became certified as a LEED AP and earned a NATE (North American Technical Excellence) certification, which provides advanced training for HVAC technicians.
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