AC Making Noise? 12 Common Sounds & What They Mean
From buzzing to banging, learn what your AC noises indicate. Portland-specific troubleshooting for when to worry vs ignore.
Serving Portland, Beaverton, Lake Oswego, West Linn, and the greater metro area since 2008.
Owner & Lead HVAC Technician
🚨 Quick AC Noise Diagnosis
Normal AC Noises:
- • Soft hum when starting/cycling
- • Gentle whoosh of airflow
- • Occasional click of thermostat
- • Quiet fan spinning
- • Normal expansion/contraction
🚨 Call Professional Immediately:
- • Loud buzzing or humming
- • Grinding or metal-on-metal
- • Hissing (refrigerant leak)
- • Banging or explosive sounds
- • Squealing/screeching
Portland Tip: Older systems (15+ years) make more noise. Our variable-speed units run whisper-quiet.
Understanding AC Noises in Portland Homes
AC units make various sounds during normal operation, but abnormal noises can indicate serious problems requiring immediate attention. After 17 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 professional service. Remember: when in doubt about AC noises, it's always safer to call us for diagnosis.
12 Common AC Noises & Their Causes
1. Buzzing / Electrical Humming
What It Sounds Like
Loud, persistent buzzing or humming, like an electrical transformer. Can be constant or intermittent.
Most Likely Causes
- • Loose electrical connections
- • Failing run capacitor
- • Bad contactor relay
- • Voltage issues
Severity
What to Do
TURN OFF AC IMMEDIATELY!
Electrical buzzing can indicate loose connections that could cause fire. Do not ignore.
Professional Diagnosis
Electrical testing required. Cost: $150-400 for capacitor/contactor replacement.
Portland Context: Our humid climate can cause corrosion of electrical connections, leading to buzzing.
2. Clicking / Ticking
What It Sounds Like
Rhythmic clicking every few seconds, or soft ticking. Can be thermostat relay or mechanical.
Most Likely Causes
- • Normal: Thermostat relay clicking
- • Normal: Ductwork expansion/contraction
- • Concerning: Failing contactor
- • Concerning: Compressor valve issues
Severity
What to Do
If clicking matches thermostat cycles (every 10-15 minutes), it's usually normal operation.
When to Worry
Constant clicking without cycling, or clicking accompanied by no cooling. Call professional.
Cost: Thermostat issues $50-300, contactor $150-400.
3. Rattling / Shaking
What It Sounds Like
Shaking or rattling noise, like loose parts vibrating. Often worse when fan starts/stops.
Most Likely Causes
- • Loose panels or covers
- • Debris in outdoor unit
- • Failing motor bearings
- • Loose fan blade
- • Unsecured ductwork
Severity
DIY First Steps
- Check outdoor unit for loose panels
- Remove leaves, twigs, cottonwood seeds
- Ensure unit sits level and secure
- Inspect fan blades for damage
Professional Service
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.
4. Hissing / Whistling
What It Sounds Like
High-pitched hissing or whistling, like air escaping. Can be continuous or intermittent.
Most Likely Causes
- • Refrigerant leak (most serious)
- • Normal: Expansion valve (brief)
- • Air escaping from ducts
- • Loose fittings
Severity
⚠️ IMMEDIATE ACTION REQUIRED
REFRIGERANT LEAK SUSPECTED!
Turn off AC, ventilate area, evacuate if smell is strong. Call EPA-certified technician immediately.
Normal Hissing
Brief hissing when AC starts/stops is normal expansion valve operation.
Cost: Refrigerant leak repair $200-500. Emergency service may be higher.
5. Squealing / Screeching
What It Sounds Like
High-pitched squeal or metal-on-metal screeching. Gets louder as speed increases.
Most Likely Causes
- • Failing motor bearings
- • Dry or loose belt (older units)
- • Worn fan motor
- • Bad compressor bearings
Severity
⚠️ SHUT OFF IMMEDIATELY
MECHANICAL FAILURE!
Squealing indicates severe bearing wear. Continuing to run can cause complete motor failure.
Repair Required
Motor or bearing replacement. Cost: $300-600 for fan motor, $1,500+ for compressor.
Portland Context: Our variable summer temperatures cause thermal expansion that accelerates bearing wear.
Additional AC Noises & Their Meanings
The remaining 7 noises with quick explanations:
6. Grinding
Metal-on-metal grinding = bearings completely shot. Shut off immediately. Cost: $300-600 motor replacement.
7. Banging/Knocking
Loose fan blade or compressor issue. Check fan secure. Professional: $200-500.
8. Whistling
Air restriction - dirty filter, collapsed duct. Easy DIY fix first.
9. Humming (Abnormal)
Failing capacitor or motor. Normal transformer hum is OK. Cost: $150-400.
10. Sloshing
Refrigerant flow (often normal) or drain backup. Check drain pan.
11. Ticking
Normal contactor relay or duct expansion. Usually harmless.
12. Vibrating
Unit not level or loose mounting. Check installation. Cost: $100-250.
When AC Noises Require Professional Service
🚨 Emergency - Call Immediately:
- Loud buzzing or electrical humming
- Hissing sounds (refrigerant leak)
- Screeching or grinding noises
- Banging or explosive sounds
- Smoke or burning smells
⚠️ Schedule Service Soon:
- Persistent rattling or shaking
- Abnormal clicking patterns
- Increased vibration
- Whistling from ducts
- System is 5+ years old
Preventing AC Noises in Portland
Monthly Maintenance
- • Listen for changes in normal operation sounds
- • Keep outdoor unit clear of debris and leaves
- • Check for loose panels or covers
- • Clean or replace air filters monthly
- • Monitor for unusual vibrations
Professional Service
- • Annual tune-up with noise inspection
- • Belt and bearing lubrication
- • Electrical connection tightening
- • Refrigerant leak detection
- • System vibration analysis
Portland-Specific Noise Prevention
- • Clean cottonwood seeds from outdoor units during May pollen season
- • Schedule maintenance before summer heat waves to catch issues early
- • Older Portland homes may have loose installations needing securing
- • Variable summer temperatures cause expansion/contraction - normal but monitor
- • Annual professional service prevents $1,000+ emergency repairs
Frequently Asked Questions About AC Noises
Buzzing usually indicates electrical issues like loose wiring, failing contactor, or capacitor problems. Turn off AC immediately and call professional - can cause fire hazard. Cost: $150-400 for electrical repairs.
Normal clicking is thermostat relay (every 10-15 minutes). Abnormal clicking could be failing contactor, capacitor, or compressor issues. If frequent or with no cooling, call pro. Cost: $150-400.
Rattling often means loose panels, debris in unit, or failing motor bearings. Check outdoor unit for leaves/debris first. Professional repair if bearings bad. Cost: $200-500.
Hissing usually means refrigerant leak - very dangerous. Turn off AC, ventilate area, call EPA-certified technician immediately. Leaks can cause poisoning. Cost: $200-500.
Squealing indicates belt issues (older units) or failing motor bearings. Grinding means bearings completely shot - shut off immediately. Professional repair needed. Cost: $300-600.
AC Making Strange Noises? Don't Ignore Them!
Professional AC noise diagnosis and repair in Portland. Same-day service for critical issues.