Buzzing, banging, squealing, or clicking: each noise points to a specific problem. Here’s what 12 common AC sounds actually mean and which ones need immediate attention.
Serving Portland, Beaverton, Lake Oswego, West Linn, and the greater metro area since 2008.
Owner & Lead HVAC Technician
Portland Tip: Older systems (15+ years) make more noise. Our variable-speed units run whisper-quiet.
AC units make various sounds during normal operation, but abnormal noises can indicate serious problems requiring immediate attention. After 18 years diagnosing AC issues throughout Portland, I’ve heard every imaginable sound, and learned which ones mean "schedule maintenance" versus "call emergency service now."
Portland’s climate and home construction create unique acoustic challenges. Our humid summers can cause expansion/contraction noises, while older homes with crawlspace installations often amplify sounds. Meanwhile, our occasional heat waves put extra stress on systems, causing premature wear that manifests as unusual noises.
This guide covers 12 common AC noises, from harmless to hazardous. Each includes what it sounds like, likely causes, severity level, and whether you can ignore it or need expert repair. Remember: when in doubt about AC noises, it’s always safer to call us for diagnosis.
Loud, persistent buzzing or humming, like an electrical transformer. Can be constant or intermittent. If the outdoor unit hums while its fan sits still, see why a condenser fan stops spinning.
TURN OFF AC IMMEDIATELY!
Electrical buzzing can indicate loose connections that could cause fire. Do not ignore.
Electrical testing required. Cost: $150-400 for capacitor/contactor replacement.
Portland Context: Our humid climate can cause corrosion of electrical connections, leading to buzzing.
Rhythmic clicking every few seconds, or soft ticking. Can be thermostat relay or mechanical.
If clicking matches thermostat cycles (every 10-15 minutes), it’s usually normal operation.
Constant clicking without cycling, or clicking accompanied by no cooling. Call professional. Also call if the ticking sounds like electrical arcing, happens while the AC is off, or started after a power surge or storm.
Narrow It Down: Note when and where it happens. Ticking only at startup is usually the thermostat relay; ticking while the fan runs points to debris in the blower. Ticking at the outdoor unit suggests compressor or electrical issues, while ticking indoors is more often the blower or ductwork.
Cost: Thermostat issues $50-300, contactor $150-400.
Shaking or rattling noise, like loose parts vibrating. Often worse when fan starts/stops.
If rattling continues after cleaning, bearings may need replacement. Cost: $200-500.
Portland Context: Cottonwood season (May) fills outdoor units with seeds that cause rattling when they get in fans.
High-pitched hissing or whistling, like air escaping. Can be continuous or intermittent.
REFRIGERANT LEAK SUSPECTED!
Turn off AC, ventilate area, evacuate if smell is strong. Call EPA-certified technician immediately.
Brief hissing when AC starts/stops is normal expansion valve operation.
Cost: Refrigerant leak repair $200-500. Emergency service may be higher.
High-pitched squeal or metal-on-metal screeching. Gets louder as speed increases.
MECHANICAL FAILURE!
Squealing indicates severe bearing wear. Continuing to run can cause complete motor failure.
Motor or bearing replacement. Cost: $300-600 for fan motor, $1,500+ for compressor. See our cost guide if replacement makes more sense.
Portland Context: Our variable summer temperatures cause thermal expansion that accelerates bearing wear.
The remaining 7 noises with quick explanations:
Metal-on-metal grinding = bearings completely shot. Shut off immediately. Cost: $300-600 motor replacement.
Loose fan blade or compressor issue. Check fan secure. Professional: $200-500.
Air restriction - dirty filter, collapsed duct. Easy DIY fix first.
Failing capacitor or motor. Normal transformer hum is OK. Cost: $150-400.
Refrigerant flow (often normal) or drain backup. Check drain pan.
Normal contactor relay or duct expansion. Usually harmless.
Unit not level or loose mounting. Check installation. Cost: $100-250.
Professional AC noise diagnosis and repair in Portland. Same-day service for critical issues.
Buzzing usually indicates electrical issues like loose wiring, failing contactor, or capacitor problems. Turn off AC immediately and call professional - can cause fire hazard.
Normal clicking is thermostat relay. Abnormal clicking could be failing contactor, capacitor, or compressor issues. If frequent or accompanied by no cooling, call pro.
Rattling often means loose panels, debris in unit, or failing motor bearings. Check outdoor unit for leaves/debris first. Professional repair if bearings bad.
Hissing usually means refrigerant leak - very dangerous. Turn off AC, ventilate area, call EPA-certified technician immediately. Leaks can cause poisoning.
Squealing indicates belt issues (older units) or failing motor bearings. Grinding means bearings completely shot - shut off immediately. Professional repair needed.
A few recent Google reviews from homeowners across the metro. We don't edit them.
Phil did a great job of carefully examining all the components of our almost 20 year old HVAC system. He took detailed photos and videos to show me the status. And he didn’t try to sell me anything else! Great experience 10/10System inspection
Efficiency Heating and Cooling came out to our home to do general maintenance on our furnace and heat pump, and also level our heat pump. Tristan was our technician. His work was efficient and thorough. He explained everything he did and told me what Efficiency H&C had to offering the way of maintenance programs. I am very happy with the services we received.Furnace & heat pump maintenance
This was my first time working with this company, and it was an amazing experience! Wayne was super personable, and explained things thoroughly and transparently. The cost was reasonable and that was with the maintenance membership program we joined. Highly recommend this business!Maintenance membership
I definitely recommend Efficiency! On this year's hottest day my AC quit working and Efficiency was able to send a technician right out. He fixed it quickly (luckily it was not a major issue) and now I'm basking in cool air. Thank you so much!AC repair
Tristan was very thorough with diagnosing the issues with my system and walked me through them so I could better understand my options for repair. We’ve had a few HVAC techs look at our system over the last decade or so, and Tristan was by far the best.System diagnosis
Jason came out for an estimate for a heat pump for my house. He listened to what I was looking for function-wise, evaluated the property, gave me a quote with 4 options, and explained the pros and cons of each. He was thorough, friendly, and not at all condescending (women, if you know, you know). Very professional.Heat pump estimate
I bought a furnace from these folks over 10 years ago and was very impressed with their work. I've had them tune up my furnace every year since and it's still the great quality and friendly service. They are dependable and knowledgeable every time I deal with them. I highly recommend them.Furnace install & tune-ups
Efficiency Heating and Cooling is a great company to work with. They gave me multiple options based on personal wants/needs with my 100+ year old house and were patient helping educate me and navigate to a decision. They were prompt, scheduling was easy, very professional, knowledgeable, and walked through with me how my system works once the job was complete. Couldn’t be happier from beginning to end and would heartily recommend them!100+ year old home
Called so many companies to find someone who did duct work beyond cleaning (I had a leak in the duct) and.so glad I got a hold of these guys. Professional, fast, and thorough. Great all around!Duct leak repair
Hire them immediately! Super fair pricing, great service. My Technician, Tristian was communicative and thorough. I tried hiring another company prior and they had me pre-pay, after tons of no shows and shoddy work followed by more no shows I demanded my money back and happily gave it to Efficiency Heating and Cooling.HVAC service
Tristan was very professional. He was on time and explained everything thoroughly. I appreciated the no pressure conversation of future work that could be done with my heating system.Heating system service
What a great experience with Efficiency. From the estimate process to installation, they made everything seamless and easy. When we hit an unexpected snag during installation, they kept us well informed about what was going on how it might affect the timeline. They got everything done and the system is working great!System installation
Two tune-ups a year, priority scheduling and 15% off repairs. No contract, cancel anytime.
Month-to-month · No long-term contract · Covers one system; add more for a discount
You'll talk to someone in our Milwaukie office, not a call center. We'll get a tech out and give you the price before any work starts.
(503) 698-5588Rather talk to someone?
