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Hearing a thump coming from your HVAC system is many homeowner’s worst nightmare, but it doesn’t have to be. Not all thumping noises in heating and air conditioning systems signal costly repairs or lengthy downtime. Though only a qualified technician can provide a definitive answer, having a few clues about what might be wrong can help narrow things down. Here’s what you need to know.

Understanding Your HVAC System

Most modern heating and cooling systems feature a series of air ducts connected to a blower motor. When something goes wrong with your system, it will usually be something wrong with the blower motor, as it contains the highest proportion of moving parts. Sometimes, though, things go wrong with ducts. Ducts can come loose, while filters can become clogged with dust, pet hair, and the other detritus of daily life.


Rattling is one of the most common sounds homeowners hear coming from their heating and cooling systems. Most often, rattling sounds mean that something in the blower motor is beginning to work itself loose, causing a part of the motor or assembly to strike against nearby duct work. If you hear rattling, you should immediately turn your system off and have it inspected. Attending to whatever is causing the rattle is often as simple as tightening connectors. If you wait until something actually breaks, repairs may be significantly more costly.


“Thwapping” (or a sound similar to a piece of rubber or plastic striking a hard surface) is another common indication of trouble, but doesn’t necessarily mean an expensive repair. Sometimes objects get caught in HVAC fan blades, striking against nearby duct work or the blower motor as the fan turns. If you’ve had your heat register covers off or recently replaced a filter, it is possible something that shouldn’t be there has gotten into the system and is gumming up the works. Most often the noise goes away as soon as the obstruction is cleared.

Clanging or Banging

Clanging or banging are also common sounds for a faulty HVAC system to make. Usually these sounds are the result of the blower motor not operating smoothly. Sometimes the motor is simply out of alignment and is banging on nearby duct work as the motor components turn. Clanging and banging can also mean something has broken within the motor itself and is banging around inside the housing. Either of these problems needs immediate attention, as allowing a broken motor to run can cause more damage. Sometimes, however, the problem is that a connection in the duct work has come loose and is being rattled by the force of air passing through it.


While not technically a “thumping” noise, screeching sounds are common enough that they deserve some consideration. HVAC systems (like the electric motors in automobiles) operate via the use of belts. A worn, cracked, or dried-out belt can produce a screeching noise. Belts are usually a quick and inexpensive fix; most often you simply need to have the belt replaced. Like the other problems on this list, however, you’ll need to attend to the problem. Once a belt actually breaks, it can damage other components of your system and require a more expensive repair.

Final Thoughts

The HVAC system in your home is crucial to keeping your family comfortable during hot and cold weather, so it pays to take care of it. By addressing signs of trouble early, refraining from running a noisy system, and investing in the services of a qualified repair tech, you can prevent more costly repairs and keep your system running smoothly for years to come.