Odd Sounds: What’s Your HVAC Trying to Tell You?

Close-up of a man trying to hear

Unlike the ones you have on deck for parties and celebrations, the noisemakers sounding off in your house right now are way less fun — as in screeching blower fans, clanking AC parts, and/or hissing air ducts. But what’s your HVAC trying to tell you, exactly? It could be a number of things, including that there’s an electrical problem, your compressor is operating normally, or it’s time to change your air filters. Keep reading to match the sound(s) you’re hearing with the messages your HVAC system could be sending your way.

1. The Blower Fan Is Busted (or About to Be)

HVAC fan
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A squealing noise soon after your HVAC system cuts on could signal the blower fan belt is going bad or that the blower motor needs lubrication. If the first possibility is the culprit, be sure to replace the faulty belt with a quickness — if it breaks completely, the blower won’t work at all, and there could be some extensive system damage.

2. There’s High Internal Pressure

You may also hear a high-pitched squealing if your compressor is experiencing high internal pressure. (Note: Sometimes, a hissing noise is an early indicator of high internal pressure; the squealing comes as the situation worsens.) The sound will only last a few seconds, but it’s warning you of danger, and your AC unit should be shut off immediately. Consult an HVAC professional for help solving this issue.

3. The AC Unit Is Working As It Should

Is your air conditioner making a humming noise? That’s just the compressor starting its cooling process. And how exactly does your AC system know when to turn on? Your thermostat will send a message that things inside are getting a little too hot for comfort and need the help of the cooling system to chill things out.

Simply put, the compressor is the part of your air conditioning that sends refrigerant where it needs to go. Located in the outdoor unit, the compressor receives the refrigerant, which has just absorbed hot air in your home, and sends that hot air to the condenser that ultimately releases the heat outside.

4. The Ductwork Is Expanding and Contracting

Ductwork spiral
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Another strange noise that’s totally normal? The airflow inside your ductwork. As heated or cooled air moves through the air ducts, it can trigger a rattling sound. Since air ducts are typically made of metal, the temperature changes occurring inside them during this transfer of air will cause them to expand and contract, making a rattling noise as they do.

Other (less normal) reasons you might hear rattling in your HVAC could be debris in your condenser unit or loose parts.

5. The Air Conditioning System Has Loose Parts

Clanking, banging noises, or buzzing sounds emanating from your air conditioning unit can be caused by a number of things, including loose parts — think detached fan blades/fan motor or disconnected wiring. Other potential reasons for hearing a buzzing noise from your HVAC could be:

  • A refrigerant leak
  • Frozen evaporator coils
  • Dirty air filters and/or condenser coils
  • A malfunctioning compressor
  • Debris in your system
  • Pests in your ductwork (e.g., bees)

The only way to determine the true source is to call in an HVAC technician for an inspection. 

6. The Thermostat Is Malfunctioning

Person adjusting the AC thermostat
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Occasional clicking sounds — like when your HVAC system turns on and off — are normal. But, a persistent clicking noise might indicate your thermostat is faulty and is causing your system to start and stop too often.

An HVAC technician can help by troubleshooting the problem to see if the thermostat needs replacing, needs new wiring, or even if it needs to be relocated. (Installing a thermostat near a window, bathroom, or door, for example, can throw off the sensors.) 

Constant clicking noises can also communicate:

  • An electrical problem (For example, a relay switch may not be working properly, and therefore, won’t turn on the compressor.)
  • A cracked heat exchanger (This can leak carbon monoxide into your living spaces.)

7. There’s a Leak Somewhere

Sealing air duct with caulk
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Besides being an early signal of high internal pressure in your compressor, an HVAC that’s hissing like a snake could also mean there’s a refrigerant leak or an air duct leak

More dangerous is the refrigerant leak, as breathing in that gas over a long period of time can potentially cause dizziness, nausea, lung damage, and shortness of breath, to name a few. It can also damage your HVAC; when refrigerant levels get too low, the AC won’t cool effectively, causing the system to overwork itself, which can also make your energy costs surge.

Air duct leaks also make a hissing sound as the air escapes through holes in your ductwork. A fairly common occurrence — about 20% to 30% of heated or cooled air is lost this way — you can DIY duct sealing or hire an HVAC technician to do it for you.

Note: Your HVAC may also give off a hissing sound if air filters are dirty because the buildup of dust and dirt can restrict airflow.

FAQ About Odd HVAC Noises

What heat pump and air conditioner noises are considered normal?

Strange sounds or loud noises coming from your AC unit don’t have to mean there’s a problem. Below are a few common AC sounds you absolutely should be hearing:

  • Humming upon startup
  • Rattling as heated or cooled air moves through ductwork
  • Fan motor blowing air through vents
  • Periodic clicking to signal start/end of heating or cooling system

Can you soundproof an HVAC system?

Yes, and it’s DIY-able! Here are a few ways:

  • Use a sound blanket to cover the compressor (the loudest part).
  • Close any air duct leaks.
  • Build a fence around your outdoor unit to absorb some of the sound.
  • Upgrade your HVAC system if necessary (newer models tend to be quieter).
  • Install silencers for your ductwork.

What is a duct silencer?

Duct silencers (AKA attenuators) are HVAC accessories used to muffle routine noises your air ducts make. Typically made of steel and customizable in shape and size to fit your system, these silencers are usually installed near fans in your ductwork. 

When to Hire a Professional

Any time you hear an unusual sound coming from your HVAC, it could signal it’s time for a tune-up and/or heat pump or AC repair. And since putting off servicing or ignoring the noises altogether could lead to extensive (and expensive) damage to your system, it’s best to hire an HVAC professional near you to diagnose and remedy the issue(s).

To help prevent any future unwanted sounds (and keep your HVAC functioning safely and effectively), remember to schedule an annual maintenance appointment. This will ensure increased efficiency and system lifespan, as well as less malfunctions/repairs.

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Andréa Butler

Andréa Butler is a writer and editor. And while she hasn't been blessed with DIY skills herself, she is adept at writing and enjoys sharing home improvement tips and pool care guides for the true DIYers out there.