Shut the door on knocking air ducts
Air ducts can make a popping or knocking sound in response to air pressure and temperature changes.
This is also known as “oil canning,” after the flexible metal of old oil cans, which makes similar sounds.
Air duct oil canning can also indicate that your ducts are too small for your furnace, or that there’s a problem with the furnace itself.
DIY solutions include changing your air filter, inspecting your dampers, or repairing and reinforcing your ducts—but a professional may be best able to help.
The wind outside is whipping. Maybe snow’s about to fall. You’re happy to be cozied up in your warm, comfy home. But when you turn the furnace on, you realize it’s not just the popcorn on the stove. Your air ducts are pop, pop, popping.
Why does my ductwork make banging noises? There are several different reasons you might hear ductwork popping when a furnace turns off or on, and several different ways to get your ducts to knock it off. Let’s take a closer look so you can get back to the important stuff: enjoying your safe haven away from the storm.
All ducts generate noise, but some sounds are cause for concern. A gentle humming or clicking is normal, but loud rattling, banging, buzzing, or whistling means you may have a problem requiring professional help. Some factors that cause excessively noisy ducts include dents or damage, loose metal parts, dirty or blocked vents, and closed dampers.
People often wonder why they hear duct noise when a furnace turns off—or on, for that matter. The change in air temperature or pressure can cause the metal ductwork to expand and contract, a process also known as “oil canning,” after the old-school oil cans that could be slightly bent and flexed.
While this isn’t necessarily a structural problem and can occur with normal operation, the noise can be annoying to some homeowners—and very loud noise that doesn’t respond to duct repair could indicate a larger issue with your furnace.
Learn why ductwork pops when your furnace turns off and how to eliminate ductwork noise.
In some cases, the popping sound may only be a specific weak spot in your ducts that needs extra support, rather than a more holistic problem. If you can discern where the oil canning is happening, the fix may be as simple as tightening the screws in that area or adding in an additional piece of thicker metal to reinforce it.
Sometimes, ductwork pops because it’s the wrong size for the specific requirements of your furnace or HVAC system. In short, if your ductwork is too small to handle the pressure your furnace can put out, it might be popping as it expands to try and handle greater airflow than it’s really designed for.
Along with being a noisy distraction, if your ducts are too small, they can affect the longevity and efficiency of your HVAC system, so it’s important to have the problem diagnosed by a professional. In this case, you may need to budget to cover air duct replacement costs.
Because of their right angles and edges, rectangular ducts are far more likely to oil can than round ones are. Unfortunately, rectangular or square ducts are much more common than round ones, though it’s possible to get round ducts retrofitted to your home.
Again, this isn’t necessarily a major problem that needs to be addressed for structural or safety reasons—but for some homeowners, the sound can be an annoyance.
In some cases, the popping or knocking sound may be coming not from your ductwork, but from the furnace itself, or popping ducts may indicate furnace issues. If you’ve ruled out all the other potential problems and are still hearing popping and knocking sounds, it may be worth having a professional look at your furnace.
No matter why your ducts are knock, knock, knocking, you probably want to get them to knock it off. Few things are more distracting than phantom popping sounds when you’re trying to cozy up on a chilly winter night.
Depending on what’s happening, there are a few different ways to get that popping noise to pop on out of your life.
As mentioned above, in some cases, oil canning may be an isolated problem only coming from a specific portion of your ductwork, in which case you may be able to reinforce the area with a more robust metal.
Insulating your ducts with fiberglass insulation or sound-dampening duct liners can also help reduce the noise, and having your ductwork professionally cleaned can not only help you find the source of the problem but also keep your system working properly for longer.
Your ductwork likely includes a system of dampers that works to help control the airflow and temperature, and sometimes, the popping sound may indicate that one of these needs to be adjusted. Opening dampers on your ductwork’s supply side may help alleviate the noise. If you don’t know how to do this, an HVAC professional can help.
It’s important to regularly change out your air filter to help ensure you’re breathing clean, high-quality air in your home—but doing so can also potentially reduce the noise coming from your ductwork system. That’s because a highly clogged filter can reduce the overall airflow, which may change the behavior of your ductwork.
If the problem seems to be coming not from the ducts themselves but from the furnace or HVAC system, it may be worth having your unit professionally serviced, just to ensure there’s nothing seriously wrong. An HVAC professional may also be able to assess your ducts and help you determine what specifically is causing the knocking sound.
Since round ducts are much quieter than square ducts—and because the popping sound may indicate weakening metal throughout your ductwork system—in extreme cases, you might consider replacing your ducts altogether to alleviate the knocking sound.
That said, this is likely to be an expensive and time-consuming project, so trying other solutions, like insulating your ductwork, may be a better first step.
As always, if you’re unsure where to turn, your local duct installer can help you decide—and knock out the knocking sound, once and for all.
The cost of professional duct repair ranges from $200 to $2,100. However, most homeowners will pay an average of $660 on this project, depending on the extent of damage.
If you are ready to head to your local hardware store and put in the work, you can save on labor fees for duct repair. However, your ductwork is a complex and vital system in your home, so it’s better to leave repairs to an expert.
You may find that hiring a pro saves you a lot of stress and problems down the road if issues with your home repair arise. Hiring an HVAC repair professional near you to help can actually save you money in the long run.
There are many reasons a furnace can make a banging noise, which also means there are a few potential solutions. The banging noise could be caused by expanding and contracting metal due to temperature changes, dirty burners, or loose ducts. To help with the banging, clean the burners, secure any loose ductwork, or check for pressure issues in the air ducts. If the noise persists, you should hire an HVAC professional for an inspection.
Vents may blow air even though the air conditioning system is off because of the pressure difference between indoor and outdoor air or drafts from leaky ducts. Check for leaks or drafts in the ductwork or around windows and doors.
While it is normal to hear some air flow from vents when the HVAC system is running, if the noise is excessively loud or unusual, it could indicate a serious issue. You need to check for a clogged filter, blocked ducts, or an oversized system. Schedule regular HVAC maintenance to help unwanted noise to a minimum.
