An air conditioner that makes a weird noise is harder to diagnose than one that makes a loud noise. The loud noises, the grinding, the banging, the screeching, covered in the previous guide, are obvious failures. The weird noises are the subtle ones. A clicking that was not there last week. A gurgling sound from the indoor unit when the AC shuts off. A whistling from the vents. A popping from the ducts. These sounds are not loud enough to make you stop what you are doing, but they are persistent enough to make you wonder if something is wrong. Some of them are normal. Some of them are early warnings of a problem that will get worse. The diagnostic task is distinguishing between the two.
The air conditioner is a system of moving parts, changing pressures, and shifting temperatures. Some amount of noise is inherent to its operation. The compressor starts and stops. The refrigerant changes from liquid to gas and back. The metal ducts expand and contract with temperature changes. The key question for any weird noise is whether the noise is new. A sound that the AC has made since the day it was installed is part of its normal operation. A sound that started last week and is getting louder or more frequent is a problem developing. The timeline distinguishes normal from abnormal.
EPA WaterSense advises that maintaining home systems prevents failures and saves energy. A weird noise from your AC is often the first sign that maintenance is needed. Investigating the noise early prevents the small problem from becoming an expensive repair.
Clicking or Ticking: Electrical Components Cycling
A single click when the AC turns on or off is the contactor, the electrical relay that connects power to the compressor and the condenser fan. The click is normal. A rapid clicking, several clicks in succession, is a contactor chattering. The contactor is trying to engage but not holding. Chattering is caused by low voltage to the contactor coil, a failing contactor, or a safety switch that is opening and closing rapidly. A chattering contactor will eventually weld its contacts together or burn them out. The contactor should be replaced. The part costs $20 to $40. The labor costs $150 to $250.
A ticking sound that continues while the AC is running, a steady tick-tick-tick, may be the expansion device, a metering valve that controls the flow of refrigerant into the evaporator coil. A thermal expansion valve, a TXV, makes a faint ticking as it modulates. The sound is normal if it is quiet and has always been present. If the ticking is new, loud, or erratic, the TXV may be failing or the system may have debris in the refrigerant that is causing the valve to stick. A failing TXV should be diagnosed and replaced by a technician.
Gurgling or Bubbling: Refrigerant Flow and Drainage
A gurgling sound from the indoor unit, especially when the AC shuts off, is usually refrigerant flowing through the evaporator coil as the pressures equalize. After the compressor stops, the high-pressure liquid refrigerant on one side of the system flows into the low-pressure side. The sound is the refrigerant moving through the coil. A quiet gurgle at shutdown is normal and has been present since the unit was new.
A gurgling sound that is loud, continuous, or accompanied by bubbling from the drain pan is a different problem. It may be the drain line partially clogged, with water and air bubbling through the restriction. The sound is similar to the last bit of water draining from a sink. If you hear gurgling from the drain, clear the drain line as described in the AC leaking water guide. Pour a cup of white vinegar into the drain to dissolve any algae or mold that is causing the restriction.
A gurgling sound that occurs while the AC is running, not just at shutdown, may indicate that the refrigerant charge is low and the system is pulling a vacuum on the low side, causing the refrigerant to flash into gas prematurely. This is a refrigerant problem that requires a technician to diagnose and correct.
Whistling or Hissing: Air Leaks and Refrigerant Escapes
A high-pitched whistling sound from the vents or the ductwork is an air leak. Air is escaping through a gap in the duct, a poorly sealed register, or a loose connection. The leak may be in a duct in the attic, basement, or crawlspace. Walk through the house and listen near each register. The whistling may be louder in one room than another, indicating the leak is in the duct serving that room. Seal the duct leak with foil tape or mastic.
A whistling sound from the return grille is often a filter that is too restrictive for the system. A high-MERV filter, MERV 13 or higher, creates more resistance to airflow than the system was designed for. The blower pulls harder against the restriction, and the air rushing through the filter media creates a whistling sound. Replace the high-MERV filter with a MERV 8 filter, which provides adequate filtration for most homes without the airflow restriction.
A hissing sound from the refrigerant lines or the indoor coil is a refrigerant leak. The hiss is gas escaping under pressure. A refrigerant leak requires a technician to locate and repair. The cost is $300 to $1,500. Do not ignore a hissing sound that you can localize to the refrigerant lines. The hiss means the system is losing charge and will eventually stop cooling.
Popping or Creaking: Thermal Expansion in the Ducts
A popping or creaking sound from the ductwork, especially when the AC first starts or after it shuts off, is thermal expansion and contraction of the metal ducts. The ducts are cold when the AC runs and warm up when it shuts off. The metal expands and contracts. The popping is the metal flexing. The sound is louder in rectangular ducts than in round ducts because the flat surfaces of rectangular ducts flex more. The popping is not a problem for the AC. It is a characteristic of the duct installation.
If the popping is loud enough to be annoying, the ducts can be reinforced. Cross-breaking the flat surfaces of rectangular ducts, pressing a diagonal crease into the metal with a duct brake, stiffens the surface and reduces the popping. Insulating the ducts reduces the temperature swing that causes the popping. Duct insulation is typically installed during initial construction. Adding it after the fact requires accessing the ducts in the attic or crawlspace.
Water-Related Sounds: Dripping and Sloshing
A dripping sound from the indoor unit is condensation falling from the coil into the drain pan. A slow drip is normal. A rapid dripping or a splashing sound may mean the drain pan is overflowing because the drain line is partially clogged. The water level in the pan is higher than normal, and the drips are falling from a greater height. Check the drain pan. Clear the drain line if the water level is high.
A sloshing sound, water moving around inside the indoor unit, is water pooled in the drain pan or in the secondary pan under the unit. The sloshing is usually accompanied by a musty smell because the standing water has been there long enough to grow mold. The drain line is clogged. Clear it. If the sloshing is coming from the outdoor unit after the AC has been off, it may be water pooled on top of the compressor or in the base pan after rain. This is normal and will evaporate.
Frequently Asked Questions
Why do I only hear the weird noise at night?
The house is quieter at night. The daytime background noise of traffic, appliances, and activity masks sounds that become audible when everything else is silent. The noise was present during the day. You did not hear it. The fact that you only notice it at night does not mean it only happens at night. It means the signal-to-noise ratio in your house is better after dark. The noise is real. Diagnose it the same way you would a daytime noise.
Why does the weird noise only come from one vent?
The duct serving that vent has a problem. A loose damper, a piece of debris, or a disconnected section of duct can create noises that are specific to one register. Remove the register cover and look into the duct with a flashlight. You may find a loose damper blade, a piece of drywall that fell in during construction, or a duct section that has separated. The problem is in the duct, not in the AC.
What are normal sounds for an AC to make?
A soft hum from the compressor, a whoosh of air from the condenser fan, a click at startup and shutdown from the contactor, a faint gurgle from the indoor coil at shutdown, and a gentle rush of air from the registers are all normal. The AC should not be silent. If the sounds have been present since the unit was installed and have not changed in character or volume, they are part of the unit’s normal operation. The question to ask is not “is it making a sound” but “has the sound changed recently.”
The Bottom Line
An AC making a weird noise may be normal or may be an early warning. A clicking that has always been there is normal. A gurgling at shutdown is normal. A popping from the ducts as they warm and cool is normal. A clicking that started last week, a gurgling while the unit is running, or a whistling from a vent are not normal and should be investigated. The distinction between normal and abnormal is whether the noise is new, whether it is getting louder, and whether it corresponds to a change in performance. A weird noise with normal cooling is usually not urgent. A weird noise with reduced cooling is a problem that is actively developing. Pay attention to the timeline. The AC that has made the same noise for years is telling you it is operating normally. The AC that started making a noise last week is telling you something has changed. Listen to it.
Last modified: July 7, 2026
