Your AC is running and suddenly you hear a screeching sound like nails on a chalkboard, or a rattling from the outdoor unit that sounds like a rock in a dryer. AC noises aren’t just annoying — they’re your system telling you something is wrong. Different sounds mean very different diagnoses.
After 20+ years diagnosing AC problems across Denver, Northglenn, Thornton, and Westminster, I’ve heard them all. This guide decodes every common AC noise, tells you the urgency level, and gives you real repair cost ranges so you know exactly what you’re dealing with.
⚡ Quick Answer: AC Noise Cheat Sheet
The 7 Most Common AC Noises (Decoded)
Let’s go through each sound in detail — what’s causing it, how urgent it is, what you can check yourself, and what it will cost to fix in the Denver area.
1. Banging or Clanking
A loud banging or clanking from the outdoor unit almost always means something is loose and physically hitting another component — most commonly a fan blade that’s loose or bent and striking the housing, a broken compressor mount, or a large piece of debris (like a rock or cottonwood clump) inside the unit.
This is one of the few AC sounds where you should turn off the system immediately. Running it while something is banging inside the outdoor unit will escalate a $200 repair into a $2,000 compressor replacement in a matter of minutes.
DIY check: Look through the top grate of the outdoor unit with a flashlight. Check for visible debris — twigs, rocks, large cottonwood clumps. If you can safely remove the debris without reaching inside, do so with the power off at the disconnect box. If nothing visible, the problem is internal.
- Fan blade or loose panel: $100–$400 to repair
- Compressor mount failure: $300–$600
- Compressor damage from running while banging: $1,500–$2,500
2. High-Pitched Screeching or Squealing
That metal-on-metal screech you’re hearing is almost always the outdoor fan motor or indoor blower motor with failing bearings. As the bearings wear out, the shaft wobbles and the metal components grind against each other — that’s your screech. Less commonly, a screeching sound can indicate a refrigerant pressure issue, but motor bearings are the culprit about 90% of the time.
Screeching motors won’t immediately destroy your system, but they will fail completely and soon. Once a motor seizes, it often takes out the capacitor and sometimes the contactor with it — turning a $300 repair into a $700 one.
Repair cost: $200–$500 for outdoor fan motor bearing service or replacement. Indoor blower motor: $400–$700.
3. Hissing or Bubbling
A hissing sound from your AC — especially from the refrigerant lines, outdoor unit, or indoor air handler — indicates refrigerant escaping from a leak point. Continuous hissing is more concerning than a brief hiss at startup. A bubbling sound means liquid refrigerant is mixing with air at the leak site.
Both sounds mean the same thing: you have a refrigerant leak. As refrigerant escapes, your system will progressively lose cooling capacity and eventually freeze up the evaporator coil. Refrigerant handling requires an EPA 608 certified technician — this is not a DIY repair.
- Leak detection, repair, and refrigerant recharge: $400–$800
- Major leak at evaporator coil (coil replacement): $1,000–$1,800
- Lance holds EPA 608 Universal certification for all refrigerant work
4. Loud Buzzing
Buzzing from your outdoor unit has two primary causes: electrical buzzing (contactor failing, loose electrical connection, failing capacitor) or mechanical buzzing (fan blade slightly out of balance, vibrating against housing). Electrical buzzing is more serious.
If your AC is buzzing but not cooling — the outdoor unit hums but the compressor doesn’t start — that’s a classic failing contactor. The contactor is the electrical switch that sends power to the compressor; when it fails, you get buzz but no cooling.
- Contactor replacement: $150–$350
- Capacitor replacement: $325 (our flat rate)
- Fan blade rebalance / replacement: $150–$350
- Electrical connection tightening: included in diagnostic
5. Rattling
Rattling is the most DIY-fixable AC noise. Common causes include loose panels on the outdoor unit (screws vibrate loose over time), debris inside the outdoor unit (twigs, rocks, even large insects), and — in Denver specifically — cottonwood buildup that clogs the fan cavity and causes vibration.
Denver-specific: Every May, cottonwood from surrounding trees fills outdoor unit coils and fan cavities. This is extremely common in Northglenn, Thornton, and Westminster. It causes rattling, reduced airflow, and can lead to overheating and shutdown. If your AC rattles every spring, plan for an annual cottonwood cleaning.
- Debris removal (DIY): $0
- Service call to clear cottonwood buildup: $75–$125
- Loose panel tightening: included in diagnostic
- Internal blower wheel cleaning: $100–$200
6. Clicking at Startup / Shutdown
One or two clicks when your AC starts up or shuts down are completely normal. You’re hearing the relays and contactors engaging or disengaging, and the ductwork expanding and contracting with temperature changes. This is standard system behavior.
Constant clicking that won’t stop, or clicking without the system starting, is a different story. That indicates a bad contactor trying to engage, a failing thermostat, or a control board issue. If you hear it clicking repeatedly without the compressor coming on, turn the system off and call us. Repair cost: $150–$350.
7. Thumping or Thudding
A thumping or thudding sound — usually rhythmic, getting worse as the system runs — indicates an out-of-balance component. The blower wheel in the indoor unit or the fan blade in the outdoor unit can become unbalanced from debris stuck inside, a bent blade from hail damage, or accumulated dirt. As the imbalance grows, it puts increasing stress on the motor bearings.
Left untreated, a thumping blower wheel or fan blade will eventually fail the bearings it sits on, turning a $200 cleaning job into a $500 motor replacement. Catch it early.
- Blower wheel cleaning / rebalancing: $150–$300
- Fan blade replacement (outdoor): $150–$350
- Blower wheel replacement: $200–$400
Denver’s Unique Noise Drivers
Denver homeowners face AC noise causes that aren’t common in other parts of the country. Three factors stand out: cottonwood season, UV degradation at altitude, and hailstorms.
Cottonwood Season (May – June)
Denver’s cottonwood trees release their seed cotton every May and into June. It blows everywhere — and it fills outdoor AC unit coils and fan cavities like insulation. The buildup causes rattling from the fan fighting through the debris, reduced airflow causing the compressor to overheat, and efficiency loss as coil fins get clogged. Clean your outdoor unit coils every May. A garden hose rinse from the inside out (fins facing outward) takes 15 minutes and prevents a service call.
UV Degradation at Altitude
At Denver’s elevation, UV radiation is significantly more intense than at sea level. Plastic fan blade tips, rubber compressor mounts, and insulation on refrigerant lines all degrade faster. Cracked fan blade tips create vibration and noise. Deteriorating mounts allow the compressor to shift, causing hum and thudding. We see this most in systems 7+ years old without adequate maintenance.
Hailstorms
Denver averages more hail events than almost anywhere in the country. Even pea-sized hail dents the aluminum fins on your outdoor condenser coil, bends fan blades, and compresses the coil fins together — reducing airflow and causing noise. After any significant hailstorm, inspect your outdoor unit.
Denver Hail Impact
hail days per year in the Denver area
Even “pea-sized” hail damages AC coil fins, which reduces airflow and system efficiency by up to 10–15%. After a hail event, check your outdoor unit and call us if you notice bent fins or new noises.
DIY Checks You Can Safely Do
Before calling a technician, run through this checklist. These are safe steps any homeowner can take that often identify or even resolve the issue.
- Turn off the system at the thermostat (not just “fan off” — set to OFF)
- Go to the outdoor unit and inspect through the top grate for visible debris — twigs, cottonwood clumps, rocks, animal nests
- Check for any bent or obviously damaged fan blades visible through the top grate
- Listen carefully: is the noise coming from the outdoor unit or the indoor air handler? (Stand near each to isolate it)
- Check your air filter — a severely clogged filter forces the indoor blower motor to work harder and gets louder
- Check that all outdoor unit panels are tightly fastened (loose screws cause panel rattle)
- If safe to do so, turn off power at the outdoor disconnect box and gently clear any cottonwood or debris from around the base and from between the coil fins
When You Should NOT Run Your AC
Some AC noises mean you should turn the system off immediately and leave it off until a technician diagnoses the problem. Running a damaged system turns small repairs into large ones — or creates safety hazards.
- Any banging or clanking from the outdoor or indoor unit
- A hissing or bubbling sound from refrigerant lines or units
- Screeching that is getting louder or more frequent
- A burning smell accompanying any noise
- Complete loss of cooling combined with a loud unusual noise
- Sparks, tripped breakers, or burning plastic smell from the outdoor unit
When in doubt, turn it off. Running your AC at 85°F is uncomfortable; running a damaged AC can mean a $2,500 compressor replacement, a refrigerant leak, or a fire. The AC can always be restarted once a technician gives the all-clear.
Repair Cost Guide for Denver (2026)
Here are current repair cost ranges we charge in the Denver metro area. All repairs include our workmanship warranty. Our flat $89 diagnostic fee is credited toward the cost of any repair.
Most AC noise problems fall in the $150–$600 range when caught early. The ones that become expensive ($1,500+) are almost always systems that were run too long after the noise started.
Real Job: Westminster Homeowner, May 2025
A Westminster homeowner called us about a screeching outdoor unit. They’d been running it for three days hoping it would “work itself out.” When we arrived, the outdoor fan motor had seized, and the run capacitor had burned out as a result. What would have been a $350 motor replacement was now a $575 repair. The unit was a 6-year-old Carrier — otherwise in good shape. Turn it off when it starts screeching.
