Inside your air ducts, two processes occur—contraction and expansion. As temperatures rise, your air ducts' metal parts heat up. These parts will then move around, producing a knocking sound in furnace vents. Fortunately, this is entirely normal and generally doesn't signal an issue.
Screw some long heavier sheet metal supports along the duct faces that are oil canning. Sometimes, just putting small dimples in the face with a hammer will make it rigid enough to stop the noise.
Common causes of a knocking noise from a heat pump include frozen outdoor coils, an unbalanced indoor fan, loose parts rattling, and compressor damage. The icing on coils forces the unit to work harder, causing vibration. Fan imbalances, loose parts, and internal compressor failures can also create knocking sounds.
Banging Sounds Coming From the HVAC Ductwork
This type of noise usually occurs in quick, short bursts. It is the result of expansion and contraction in the duct walls as internal temperatures rise or fall. This type of noise can also be the result of pressure changes within the ducting.
From what you are describing, that's normal thermal expansion of the ducts. When the heat is on, they raise in temperature and grow slightly, shifting around. When the heat turns off, they cool and shrink, shifting around. All this shifting causes popping, but it's completely normal.
Inside your air ducts, two processes occur—contraction and expansion. As temperatures rise, your air ducts' metal parts heat up. These parts will then move around, producing a knocking sound in furnace vents. Fortunately, this is entirely normal and generally doesn't signal an issue.
Water hammer happens when a valve suddenly closes, forcing all the water flowing through the pipes to collide into the closed valve. That sudden, violent collision creates shockwaves that shake your pipes (creating the annoying banging noises you hear).
It's normal to hear a low humming sound as air moves through your vents. Most of the time, you probably won't even notice it. However, if that sound becomes disruptive or if you hear more concerning noises (such as rattling, clicking, banging, hissing, or grinding), there might be a problem with your HVAC system.
There is usually a simple explanation for why your walls are making a knocking sound. Most often, it's your home's plumbing system. It may be due to fluctuating water pressure issues, loose plumbing pipes, or a faulty valve.
If your furnace is making a clicking noise, this is a sign of an issue occurring during the heating cycle. When you hear clicking from your furnace, you should contact an HVAC professional immediately, as this strange sound may indicate a faulty igniter or gas valve. Issues with these parts can result in a gas leak.
The blower is the “fan” unit, and it's usually located indoors. If it's unbalanced it will create a banging or knocking noise as the fan spins. Other out-of-balance or loose parts. If the sound is more like a clanking, it could mean the compressor itself is loose, or parts inside the blower unit are hitting each other.
If the pipe is secure and simply close to a wall, joist or other pipe, consider putting a small amount of cloth, foam or felt between the wall and pipe to remove the vibration. For pipes that have come loose, it can be much more difficult, especially if they're beneath the floorboards.
Possible Causes of Ductwork Noise
The most common reasons to hear noise in your HVAC ducts include: Air Pressure: A buildup of pressure can cause rumbling sounds. It can also cause the metal to expand and contract.
Check the dampers on the supply side of your ductwork and try opening them up one at a time to see if this reduces the popping and banging noises. You can also adjust the actuator inside of the duct dampers up or down to reduce the duct noises. If you don't know how to do this, contact your local HVAC technician.
The duct silencers enable maintaining proper acoustic parameters in the rooms – even reducing noise in the rooms which require a high degree of acoustic comforts, such as e.g. sleeping rooms or reading rooms. The noise can be generated not only by the flow of air through the ducts.
When heat hits the cold metal ducts, it causes expansion. When the heat cycle finishes, the ducts start to contract which may produce a loud bang. While these noises from your ductwork can sound concerning, there's usually nothing wrong with your ducts or ductwork system – it's just a normal sound.
If you hear any clicking, buzzing or tapping sounds in the wall, you may be facing a potential termite invasion. Termites are hard to spot and often go unnoticed, causing significant amounts of damage in homes each year.
Ever noticed those knocking drain noises when hot water drains out of your sink or bathtub? It's the drain expanding. That's it, that's all.
Your homeowner's insurance won't cover cleaning, repairing or replacing your ductwork. What does a home warranty cover when it comes to AC units and ductwork?
Duct replacement costs typically range from around $20–$60 per linear foot, based on our team's market research. For example, if your home has 100 feet of ductwork, a complete replacement can cost between $2,000 and $6,000.
Ductwork has a maximum lifespan of 20-25 years, and starts to deteriorate around year 15. If your ductwork is more than 15 years old, you may need to have the ductwork inspected to see if it is time to replace it. If it is older than 25 years, you almost certainly need to replace the ductwork.