Air conditioners aren't silent even when they're brand new, so an air conditioner humming noise could be perfectly normal. If you hear a new humming noise or your air conditioning isn't working properly, you likely have an issue that needs to be repaired. Researching the source of the humming noise and doing some troubleshooting can help you figure out the problem and decide if you need to call an HVAC professional.
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Loose Internal Components
Air conditioning units have lots of internal components and moving parts. The unit vibrates a lot, which can cause parts to become loose. That often results in a humming or buzzing sound. Loose parts not only make loud noises but they can also cause internal damage or decrease the efficiency of the air conditioner. During a regular HVAC tune-up or service call to diagnose the problem, the technician will check and tighten parts to prevent this from happening.
Malfunctioning Condenser Fan Motor
Your condenser fan motor could be on the fritz if you hear a new humming sound. If the motor seizes, it still gets power and tries to turn but can't spin. This could be due to a mechanical issue, loose parts, or debris stuck in the fan. That struggle makes a loud humming sound. Since the fan isn't turning, the central air won't be able to cool your home, so this could be the issue if your A/C doesn't seem to be working correctly.
Damaged Contactor Relay Switch
Your outdoor air conditioning unit has a contactor relay switch inside that helps turn it on when your house warms up and needs to be cooled. It's a low-voltage switch that your thermostat controls. If this switch goes bad either mechanically or electrically, it can create a humming sound in your outside unit. This repair is best left to an HVAC technician.
Frozen A/C Unit
The coils in your air conditioner can freeze even when it's hot outside and you're running it nonstop. A frozen air conditioner can result when it gets dirty or your refrigerant is low. Frozen coils can cause a humming sound. When the coils freeze, the A/C can't cool your home properly. It can also do major damage if you keep the system running without letting it thaw.
If the unit is frozen, shut it off to let the ice thaw. Once it thaws completely, turn just the fan on for a few hours to help dry out the system. Check your furnace filter to see if it's clogged and change it if it's dirty. A clogged, dirty filter can contribute to the frozen coils.
Turn the air conditioner back on and keep an eye on it to see if it freezes again. If it continues freezing, have an HVAC professional inspect it to find and fix the root cause.
A/C Electrical Issues
Disruptions to the electrical flow can cause a humming sound in your air conditioner. One example is loose wiring, which can create an electrical arc and cause a buzzing or humming sound. Loose wiring can cause an electrical fire, so it's important to have the unit inspected by an HVAC professional to see if this is the problem.
If you notice the buzzing sound inside your home, it could be a circuit breaker issue. Sometimes, it can become overloaded but doesn't trip. In this case, it could create a buzzing or humming noise. An electrician or HVAC professional can help with this issue.
Handling a Humming Noise
Before you do any work or investigation on your air conditioner, shut off power to the unit at the breaker. This prevents the fans and moving parts from turning on while you're working. Shutting off the unit also prevents additional damage if you have loose parts or other issues that could break other parts.
Most humming noises require a professional fix. An HVAC technician can accurately diagnose the problem and fix it before something more serious happens. Note all of the specific noises and other symptoms you noticed, such as the system not cooling correctly, to help the technician narrow down the problem.