SMO Energy

What to Do If You Hear Strange Noises Coming From Your Heating System

In an ideal world, the appliances that make our homes comfortable places to live would operate smoothly and soundlessly. If you are concerned that your furnace is loud or that your gas heater is making noise, it is a good idea to determine the source of the noise and whether it is a “normal” sound or a sign that something is going wrong with your heating system.

Get to know some strange sounds a heating system might make and what they mean for the state of your equipment.

9 Strange Noises Your Heating System Might Make

An oil or gas heater making noise is not always a cause for concern. Knowing which sounds are all part of the normal operation of your furnace, heat pump or boiler and which ones are a reason to call in a technician will help you feel more at ease as a homeowner. Being able to identify strange sounds will also help you take action quickly, potentially avoiding the need for an expensive HVAC repair or even replacement.

Why Does My Heating System Make a Noise?

Some of the reasons why your heating system might be making strange noises include:

  • An issue with fuel ignition
  • Clogged or dirty air filters
  • Loose ducts
  • A loose blower fan
  • Cracked heat exchanger
  • Sediment build-up (in a water heater)
  • Worn out fan belt
  • A leak

More likely than not, your furnace or heating system will make a specific type of noise to alert you to a potential problem. Some of the more common noises you might hear from your heating system or hot water heater include the following.

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Strange Noises - Pilot Light

1. Furnace Banging

If you hear a loud bang from the furnace when it kicks on, it could be because of an issue with ignition. When a furnace gets the signal from the thermostat to begin a cycle, the gas valve opens. The ignition system or pilot light should ignite the gas right away. 

But when there is a problem with the ignition system, a small amount of gas can build up in the combustion chamber. When ignition does take place, there is often enough gas built up in the chamber to create a banging noise. The sound can travel throughout the house by way of the ducts. 

Several things can affect the ignition of gas in a furnace. If there is a pilot light, a weak flame might prevent the gas from igniting quickly. Furnaces that have electronic ignition might have an issue with the ignition system. Additionally, the burners might be dirty or clogged, or the gas pressure might be low enough that ignition does not take place until there is a significant amount of it built up in the chamber. 

Problems with the ignition system in a furnace should be looked at by a trained technician, and the sooner, the better in most cases.

Another reason your furnace might make banging noises is due to contraction and expansion of the ductwork. The ducts that send warm air throughout your home are made of thin metal. The metal can expand when heated and will contract as it cools down after the furnace cycles off. As the metal ducts expand, they typically make a banging or booming noise. Banging ductwork usually is not a cause for concern, but there are things you can do to reduce the noise. You can replace rectangular ducts with round ones or replace very thin metal with thicker metal.

2. Furnace Whistling

Furnace whistling occurs for the same reasons whistling in humans occurs — something is interfering with the flow of air. When you whistle, you typically make an effort to produce the noise by pursing your lips as you exhale. When a furnace whistles, a blockage, such as an air filter that needs cleaning or replacement, is responsible for the sound. Other possible causes of a whistling sound in your furnace include a blocked return, objects in the ductwork and closed vents or dampers. You might be able to solve the sound yourself by cleaning the ducts, filter and return and by opening up the vents and dampers.

In some cases, the cause of the whistling noise is something a technician needs to fix or something that can only be fixed by replacing the furnace. For example, a whistling noise can occur when your furnace is too big for your ductwork or too big for your home. A whistle can also be a sign that there is an issue with the blower or burners in your system.

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3. Furnace Clunking

When your furnace is functioning properly, it will not make clunking or knocking noises. One of the reasons a furnace could be making a knocking noise is an issue with the fan. For example, if the blower fan becomes misaligned, the blades can hit the walls or other parts of the furnace. Another cause of a clunking or knocking noise in a furnace could be a fan belt that has cracked or split and is hitting other materials as it rotates.

If your furnace goes “clunk clunk” at any time, your best option is to turn it off and call in a technician to take a look.

Strange Noises - Carbon Monoxide

4. Furnace Rumbling

When your furnace powers on to complete a cycle, you might hear a gentle woosh or roaring sound. But if that gentle roar turns into a low rumble, it can be a sign of trouble. If the rumbling is loud and continues even after the furnace has completed a cycle and switched off, it is most likely due to a problem with the burner. Oil or gas might remain in the combustion chamber once the furnace cycle completes. The fuel continues to burn, which accounts for the furnace making the rumbling noise you’re hearing.

A rumbling sound is difficult to ignore and is something that demands your immediate attention, as the extra fuel in the combustion chamber can contribute to high levels of carbon monoxide in your home. Make sure the furnace is turned off and call a technician for assistance right away.

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5. Furnace Humming

For the most part, a humming noise coming from your furnace is not a cause for alarm. Many furnaces hum or “whir” when the burner ignites. Sometimes, a clicking noise follows the hum. If the furnace is newer, the transformer might hum as it adjusts the current that flows to the appliance.

There are times when a humming sound can be a cause for concern, however. If the furnace inducer motor makes a loud humming sound throughout its operation, not just at the beginning of the cycle, it could be a sign that something is going wrong with the motor. Another cause of a loud hum is a fan that is wearing out. Pay attention to the volume of the noise and when it occurs. If it is happening all during the cycle, you might want to call in a technician to take a look at your system.

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6. Furnace Screeching or Scraping

If you hear a screeching sound from the furnace or a scraping sound, as if a piece of metal is being dragged across another piece of metal, that might be exactly what is happening. The blower fan might have come loose, and the blades might be dragging across the casing of the furnace. A scraping noise can also be a sign that other parts of the furnace have loosened. If you are not sure of the cause of the noise, turn the furnace off and have a technician come and take a look at it.

Strange Noises - Sizzling

7. Hot Water Heater Hissing, Popping, Crackling and Sizzling​

Furnaces aren’t the only heaters that make noise. Your home’s hot water heater can also be pretty noisy. It might make popping noises, hissing sounds, crackling noises and sizzling sounds: 

  • Popping or knocking sounds: Your water heater might go “pop, pop, pop” when there is a lot of sediment built up in it, or it might sound as if the hot water heater is knocking. If you have hard water, calcium and other mineral deposits can build up in the heater’s tank. The popping noises are produced by water boiling underneath the sediment. Once you start hearing popping on a regular basis, it might be time to flush the heater to clear away any buildup.
  • Crackling: When there is condensation around the water heater’s burner, it is common to hear a crackling noise. Usually, crackling is not a cause for concern.
  • Hissing: A water heater might start to hiss for the same reason a tea kettle whistles when it comes to a boil. The temperature of the tank might be too high, causing pressure to build up inside. The excess pressure pushes open the temperature and pressure relief valve, producing a hissing sound. A buildup of sediment can also increase pressure in the tank, producing a hissing noise. If you hear a hiss from your water heater, have a technician take a look at it to determine the cause.
  • Sizzling: When you hear your water heater sizzling, it is often because the water is leaking, usually falling onto a burner. Along with a sizzling sound, another sign that your water heater is leaking is a pool of water at the base of the heater.
Strange Noises - Loose Screw

8. Heat Pump Loud Vibration

When all is well with your heat pump, it should be pretty quiet, about as loud as a dishwasher. If you start to notice a loud vibration noise or humming from the heat pump, there are things you can do to correct the problem. Often, loud vibration noise is caused by an uneven surface. You can try to put a rubber pad under the heat pump to absorb the sound and to create a more level surface. Vibrating noises can also happen when there are screws or other parts loose in the heat pump.
Strange Noises - Rattling Duct

9. Rattling Ducts

Rattling air ducts are not usually a cause for concern, as they do not indicate a larger issue with your heating system. A rattling noise from an air duct can mean something is loose about the duct, such as the joints, and that you are losing heat from it. In some cases, the duct might be smaller than the vent opening, which causes it to move back and forth and rattle as air passes through. 

How to Troubleshoot Noises in Your Heating System

Knowing what sounds are normal for your system is the first step when troubleshooting. If your furnace or heat pump usually produces a low, gentle hum, you do not have anything to worry about. But if your system is suddenly rattling, banging, clanking or whistling, you are going to want to figure out what is going on.

To protect your family and home, it is a good idea to call in a trained technician to evaluate the problem, especially if the noise is one you have not heard before. The technician will take a look at what is going on, diagnose the problem and recommend the best next steps, whether they are to repair the system or replace it.

Quick Solutions for Silencing Your HVAC System

In some cases, a noisy heating system can be silenced with a quick fix or two. Some of the things you can do at home to help your furnace or heat pump operate more quietly include:

  • Change the air filter: The filter in your furnace traps dust, hair and pet fur and keeps them from traveling through the ducts and circulating throughout your home. After months of continuous use, the filters can become pretty dirty and clogged. If you hear whistling from your furnace, you might try changing the air filter. A clogged filter can prevent air from flowing through the furnace freely. As a general rule, it is a good idea to change the filter at least once a season.
  • Request ongoing or preventative maintenance: Like your car or a bicycle, a heating system needs regular tune-ups and inspections to make sure everything is operating as it should be. During a tune-up, a technician will adjust and clean parts of the furnace, helping to improve its functioning.
  • Check dampers: If the vents or dampers are closed in certain areas of your home, airflow can be restricted, leading to a whistling noise from the furnace. Take a look at the vents around your house and slide them to the open position if needed.
  • Check ducts: Air ducts can produce a range of noises, from rattling to banging. Inspect your ductwork, looking for loose joints, dings or dents, or lots of space between the edges of the ducts and the vents. If you notice anything loose, you can try to tighten it or seal it with duct tape.

When to Call for Furnace Repair or Replacement

Although rattling, whistling and humming usually are not causes for concern, some noises should not be ignored. If your furnace produces a loud “bang” or “boom” or starts rumbling, screeching or knocking, it might be time for a repair or even a replacement, depending on its age and overall condition. Switch the furnace off and call SMO Energy if you hear any of the following from your heating system:

  • A bang
  • A boom
  • Knocking
  • Screeching
  • Rumbling
  • Loud, persistent humming

One of our trained technicians will come out and inspect your system to figure out what is going on. Based on what we find, we might recommend repair or replacement of your system. Once your furnace is fixed or a new one is installed, we recommend an annual service agreement, which covers your equipment in case it needs repair. The service agreement also includes an annual inspection and tune-up.

Don't Ignore the Signs of a Failing HVAC System​

When your heating or HVAC system begins to make strange noises, it is often trying to tell you something. Even if you can get used to the screeching, whistling or persistent humming, it usually is not the best idea to do so. Audible warning signs that something is going wrong with your heating system should not be ignored, as they can point to a serious problem with your equipment. Without attention or a repair, those problems could damage your home or hurt your family.

Contact SMO Energy if your furnace, heat pump or HVAC starts acting funny and making noise. We repair and replace heating equipment across Southern Maryland. We are happy to investigate your system and let you know what you can do next.