What Noise is That? 10 Common HVAC Noises Explained

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Posted October 11, 2019

Is your HVAC system calling out to you? Could this be a wake-up call? Pretty much all home systems make some noises at certain times—the dishwasher spraying water on dishes, your refrigerator running, and so on. We get used to these. However, we can usually tell when a noise sounds unfamiliar. Since our HVAC system can make many different sounds, and since many of us take their continual operation for granted (we shouldn’t, though), it would be a good idea for us—owners and renters alike—to know more about this topic. In this article, we’ll begin by describing some of the common noises you might hear coming from your HVAC (meaning heating, ventilation, and air-conditioning) system. Then, we’ll explain what might be happening and what you should do next.

First, though, we’ll go over the key components of an HVAC system so that you have a kind of “roadmap” to follow here.

The HVAC System

An HVAC system keeps the climate comfortable in your house, apartment, or workplace. It is comprised primarily of the heating and (often) cooling sources, as well as the ductwork that circulates the warm or cold air and a control unit.

Heating Sources

• Furnace: There are gas (the most common), oil, and electric furnaces. All have similar parts, including a burner (gas or oil) or heating element (electric) a blower, a heat exchanger, a filter, a plenum, and various other parts.
• Boiler: Like a furnace, a boiler heats your house. However, it does so by distributing heated water through tubes installed in your house.
• Heat Pump: “For climates with moderate heating and cooling needs, heat pumps offer an energy-efficient alternative to furnaces and air conditioners (by using) electricity to move heat from a cool space to a warm space.”

Cooling Sources

Heat pumps can also meet cooling needs in moderate climates. However, the most common cooling source is air conditioning (AC)—which comprises both outdoor and indoor AC units.

AC works by using chemical refrigerants (hydrochlorofluorocarbons or HCFCs) that convert from gas to liquid and back again quickly. Here too, there is a heat transfer process involved.

An air conditioning system consists of an evaporator, a compressor, a condenser, and an expansion device. The part inside the home where the refrigerant evaporates is the evaporator. Indoor fans blow air across the evaporator’s coils.

Ductwork

For a heating and/or cooling system, the ductwork is simply a “transit” mechanism to disperse the heated or cooled air throughout the house.

Thermostat

A thermostat controls a home’s interior climate by regulating the temperature. Some thermostats are programmable and can be quite sophisticated in what messages they can send to the HVAC system.

Common Noises Made by the HVAC System—And How to Address Them

Would you believe that there is a solution to each of the common HVAC noises you might be hearing at home? There most certainly is. However, be forewarned that the solution might involve some amount of complexity and cost.

Still, if you have any doubts or worries about what’s going on, it’s much better to call an HVAC technician now than wait until the problem gets worse. You’ll understand why after reading more of this article.

And here’s a great troubleshooting guide in case you need more information along the way.

Squealing or Screeching

This can signal a bad fan belt in the blower. If it is, that belt should be replaced. The sound also might mean that the motor needs lubricant—which you may or may not be able to do yourself depending on the furnace make and model.

It does take a special type of lubricant, though (not an ordinary household type). When in doubt, call an HVAC technician.

“Thwapping”

This sound could mean that something is stuck in the blower’s fan blades. If this common noise comes and goes with airflow, that’s an easy way to tell if this is the problem.

While this situation might not be critical to the ongoing functioning of your furnace, the obstructing object might be creating unnecessary wear and tear on the unit.

Rattling (Inside the House), Clanking or Banging

Like “thwapping,” these sounds could be related to the furnace’s blower assembly or motor, and are likely due to something being either loose or broken.

If you hear louder banging sounds when the blower is running, shut the system off until you can get help. At this point, something has broken or gotten disconnected.

Popping

If the sound is coming from your ductwork and the heat hasn’t been on for a while, this is a common noise. It’s the sound of cold metal ducts expanding due to the rush of warm air.

If the popping comes from another source or doesn’t go away, then it might be time to call a technician to find out for sure what’s going on.

Buzzing

Is this common HVAC noise coming from your AC unit? If so, it could be some sort of compressor malfunction. It also might be a refrigerant leak that causes the unit to freeze. Or it might be just a loose part vibrating. You should have it checked out though.

Frequent Clicking

If the clicking is coming from the AC unit, most of the time it’s either the outside compressor or the inside air handler. There are several possible causes, which can easily be diagnosed by a technician.

Is the clicking coming from the furnace? If the weather is getting colder and you turn the heat on for the first time in a while, clicking should be a welcome sound. It tells you your system is working.

However, if the clicking continues and is coming directly from your furnace, that could be a sign of an ignition system (pilot light or electronic ignition) malfunction. It might get worse—see below.

Booming

The booming sound occurs when the furnace is attempting to light (or relight) from a pilot light or electronic ignition—but with little success. So there’s an increasing gas build-up within the combustion chamber that will suddenly ignite all at once.

Needless to say, this is not a desirable situation; you should call an HVAC technician. The problem was likely caused by a build-up of carbon particles. The chamber along with the flue should be inspected and cleaned professionally.

High-Pitched Whistling or Screaming

This is a warning sound. If it (or a similar sound) is coming from your AC compressor, you need to act quickly and turn electrical power to the unit OFF. It might switch off on its own—which is good. Leave it alone until you can get help.

This also could signal a dangerously high refrigerant pressure. It might even mean that your compressor is failing. Again, seek professional help.

It might be an internal leak, too—though this is more often associated with a hissing noise (see below).

In any case, you need to call your HVAC technician as soon as possible. Don’t turn the unit back on until it has been looked at and/or repaired. Refrigerant leaks can be quite dangerous to people, animals, and the environment.

Hissing

Is the hissing sound coming from the furnace or the AC compressor?

• If it’s coming from the AC compressor, this could mean there’s an internal refrigerant leak. It also (as discussed above) could mean dangerously high internal pressure. You should disconnect the power and call a technician.

• If it’s coming from the furnace (or seems to be), most likely it is a leaking air duct—which will require professional repair. If it’s coming from the furnace itself, it might be some sort of leak that should be checked by a technician.

Rattling (From the Outside Compressor or Fan Unit)

Another common HVAC noise is rattle. This could mean that your compressor is failing. It also could a pulley with failing bearings. Or it could be a worn-out compressor clutch. Anyone of these concerns warrants the attention of an HVAC technician.

What About Boilers? Are They Different

You might be wondering if you will hear different noises from a boiler. Well, for one thing, they can make a lot of different noises: clunking, banging, whistling, and gurgling are just a few.

Like furnaces, boilers can be fueled by gas or oil. Unlike furnaces, though, boilers involve water—which can leak, have scale (mineral deposits) build-up, develop trapped air, and various other concerns.

Clunking and banging noises tend to occur when scale accumulates on the heat exchanger. According to 24/7 Home Rescue, “This blocks the flow of water and causes some water to boil, steam and expand,” as in a kettle.

Boilers also whistle, which could be an effect of either kettling or trapped air in the system. And if there’s gurgling, this could also be caused by trapped air—or sediment blockage. It might even be the result of a frozen pipe.

It should be clear that an HVAC system provides us with many common sound cues as to what’s going on with its operation. Some sounds point to hardware, while others point to various fluids. Again, an HVAC technician could say for sure what it is.

Note that some potentially dangerous escaping fluids make virtually no noise at all, and one of these is the natural gas that heats your home.

What to Do If You Smell Natural Gas (Near the Furnace or Anywhere Else)

If you have a gas leak, you probably won’t hear a noise, hissing or otherwise—at least not at first. But you will smell it.

Natural gas has a distinct odor. If you haven’t smelled it before, it’s sort of a sulfur odor, like rotten eggs. In its natural state, natural gas is odorless, so this easily discernible smell (mercaptan) is added specifically to warn people of leaks.

First things first. If you smell gas, leave the house, making sure that family and pets get out as well, and call the gas company or 911 immediately.

Do not inspect the furnace (or other gas-fueled appliances) yourself in an attempt to find the leak. Do not touch any light switches or appliance cords. Just leave.

Someone from the gas company should arrive soon after you call—hopefully, able to find the source of the odor and take care of it quickly. Then you can have an HVAC technician come by at a later time to check things out.

What HVAC Technicians Do

No, wait! You left out one noise that I keep hearing ever since the HVAC technician came by the other day. It’s sort of a quiet blowing noise coming through the HVAC vents throughout my house. Is something else wrong now?

Hmm… Could this be the sound of an HVAC system that’s back to 100% function due to the work the technician did during her or his visit? Perhaps this sound is one you haven’t heard for a while, but we’d bet it continues for quite a while now.

Continues, that is, until you’re due for your next HVAC servicing—which should happen on an annual basis—to lubricate as needed and replace worn-out parts and, generally, to ensure that all parts continue to perform as they should.

Typical responsibilities of an HVAC technician, by the way, include:

• Understanding and applying local HVAC codes professionally on each job
• Installing, cleaning, and maintaining the system
• Doing the layout for, designing, and installing low-voltage wiring related to HVAC
• Inspecting and testing the unit and its parts
• Discussing malfunctions with the owner
• Repairing or replacing parts
• Recommending maintenance for better performance
• Collaborating with sales and engineering to develop product definitions responsive to customer needs and market opportunities.
• Keeping records of the work performed for office files and billing purposes

So you can understand how well qualified these professionals are. Why risk the do-it-yourself route to HVAC repair when you can call a trained technician? Remember, winter isn’t that far off …

That’s It … Until We “Hear” More

Your HVAC system is important to the comfort level in your home, especially during extreme outdoor temperatures. If we forget about it most of the time, that’s good—as long as its good functioning is the result of regular inspections and servicing.

But we must not forget the HVAC system altogether since something will go wrong eventually, and when it does, it could well be disastrous.

Any system with moving parts, whether mechanical or chemical, goes through a lot of wear and tear (often very quietly) and needs regular maintenance to continue operating well.

Have you had your annual visit from an HVAC technician yet? No? Why not?

Go get your phone and make an appointment right away. Otherwise, you might find yourself calling when something breaks at the most inconvenient possible time!

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