Why is my AC running but not cooling the house?

The most common causes are a dirty air filter restricting airflow, a failed run capacitor preventing proper compressor or fan operation, low refrigerant charge from a leak, a frozen evaporator coil, or a failing compressor. Start with the filter and capacitor as they are the easiest to diagnose and most frequently the cause.

How do I know if my AC capacitor is bad?

Signs of a bad capacitor include: the outdoor unit hums but the fan does not spin, the compressor runs but the condenser fan does not (or vice versa), the unit struggles to start and trips the breaker, or the capacitor appears visually swollen or bulging. Confirm with a capacitor meter - a reading more than 6% outside rated MFD means replace it.

Can I run my AC if the refrigerant is low?

Running an AC with low refrigerant stresses the compressor, which operates outside its design parameters without proper cooling and lubrication. Extended operation with low refrigerant can cause permanent compressor damage - the most expensive repair in HVAC. If you suspect low refrigerant, have a licensed technician diagnose and repair the leak before adding refrigerant.

Why does my AC freeze up?

AC systems freeze when the evaporator coil gets too cold, which happens from insufficient airflow (dirty filter, closed vents, low fan speed) or low refrigerant charge. The ice insulates the coil and eventually blocks all airflow. Turn the system off, let it thaw for 2-4 hours with the fan on, then diagnose the root cause before restarting.

Home>Resources>AC Making Noise

HVAC TROUBLESHOOTING

AC MAKING NOISE

Banging, squealing, rattling, humming, clicking - each sound from your AC or furnace points to a specific cause. Use this guide to identify the sound and find the fix before it becomes a bigger problem.

Diagnose by Sound Type

Sound	Most Likely Cause	Urgency
Loud banging or clanking from outdoor unit	Loose or broken compressor mounting, broken fan blade, loose panel	Stop unit immediately
High-pitched squealing or screaming	Failing blower motor or condenser fan motor bearing	High - motor failure imminent
Rattling from outdoor unit	Loose screws, debris inside unit, loose refrigerant lines	Moderate - check soon
Loud hum from outdoor unit, no start	Failed capacitor - compressor trying but can't start	High - replace capacitor
Clicking repeatedly at startup or shutdown	Normal relay click is fine; repeated clicking = control board or contactor issue	Moderate
Bubbling or gurgling sounds	Refrigerant flow noise (normal) or air in refrigerant lines (undercharge)	Low unless paired with warm air
Whistling from vents or air handler	Dirty filter, high static pressure, undersized return duct	Low - address soon
Loud vibration from air handler cabinet	Loose access panel, loose blower wheel, vibrating refrigerant line	Low

? Stop the Unit for These Sounds

Turn off your AC immediately if you hear: loud banging or clanking (broken fan blade or compressor), metal-on-metal grinding, or a continuous screaming/shrieking noise. These indicate mechanical failure that will cause further damage if the system continues to run.

Banging or Clanking - Check Immediately

Turn the system off at the thermostat. Go to the outdoor unit and look inside through the fan grille - check for a broken, bent, or separated fan blade. A broken blade hitting the unit housing creates a loud clank with every rotation. Also check the access panel - if a screw is missing, the panel vibrates and bangs. If the noise comes from deep inside the compressor housing (even with the fan not running), the compressor itself may have a broken internal valve or mounting bracket - requires professional diagnosis.

Squealing - Bearing or Belt Failure

High-pitched squealing from the outdoor unit typically means a failing condenser fan motor bearing. The bearing is dry or worn and the motor is about to seize. Listen carefully - the squeal should clearly come from the fan motor location, not the compressor. Blower motors in the air handler can also squeal from failing bearings. If a belt-drive blower is involved (older commercial units), a worn or misaligned belt squeals. Lubricate accessible bearings if they have oil ports, but most modern motors are sealed and the fix is replacement.

Rattling - Find the Source

Rattling is often something loose: (1) Check all access panel screws on both indoor and outdoor units. (2) Look inside the outdoor unit for debris - leaves, sticks, or cottonwood seeds accumulate on the bottom and rattle when the fan runs. (3) Check refrigerant lines where they enter the building - lines that vibrate against sheet metal make a rattle that sounds like it's coming from inside the unit. (4) Check the outdoor unit pad - if the unit is vibrating on an unlevel concrete pad, the entire unit rattles. Level it and add vibration isolation pads if needed.

Humming With No Start

A loud hum from the outdoor unit with no actual startup (fan not spinning or spinning slowly, compressor humming) is a classic failed capacitor symptom. The motor windings are energized but without the capacitor's phase shift, the motor cannot develop starting torque and just hums. The thermal overload in the motor will trip after about 30 seconds, stopping the hum - then the cycle repeats when it cools down. Replace the run capacitor before this cycling destroys the motor windings.

Whistling From Vents

Whistling from supply or return registers almost always indicates high static pressure - the system is forcing air through a restriction. Common causes: dirty filter (most common), registers that are partially closed or blocked, a return duct that is undersized for the air handler's airflow, or a dirty evaporator coil restricting airflow. Check and replace the filter first. If the filter is clean and whistling persists, measure total external static pressure - it may be a system design issue.

Parts Commonly Needed

Condenser Fan Motor

For squealing - match HP, RPM, rotation, and shaft size exactly

Find Fan Motor

Condenser Fan Blade

For banging - match blade diameter and hub size

Find Fan Blade

Run Capacitor

For humming - test before ordering; match MFD and voltage

Find Capacitor

Blower Motor

For indoor squealing or rattling blower wheel

Find Blower Motor

Vibration Isolation Pads

For units vibrating loudly on concrete pads

Find Pads

Frequently Asked Questions

Why is my AC making a loud banging noise?

Loud banging or clanking from the outdoor unit usually indicates a broken or separated condenser fan blade striking the unit housing, a loose access panel, or in more serious cases, a compressor with broken internal components. Turn the unit off immediately and inspect the fan blades through the top grille before restarting.

Is it normal for AC to make noise when turning on or off?

A single click at startup and shutdown is normal - that is the contactor relay switching. A brief rattling sound as the compressor starts and stops is also normal (compressor vibration during the brief unloaded start). What is not normal: repeated clicking without startup, loud banging, continuous squealing, or any sound that is new or getting worse over time.

My AC makes a bubbling sound - is that a refrigerant leak?

A subtle gurgling or bubbling in the refrigerant lines is often normal liquid refrigerant flowing through the metering device. However, if the bubbling is accompanied by reduced cooling, warm air, or you can see the suction line (the larger copper line) is not cold and sweating as usual, the system may be low on refrigerant. Have a technician check pressures.

Related Resources

condenser fan motor condenser fan blade run capacitor

Menu

AC MAKING NOISE

Diagnose by Sound Type

Frequently Asked Questions