Your HVAC: What’s Normal and What’s Not

What's that noise? It might be your air conditioner. Whether it's a click in the night, a constant dull hum, or a sudden high-pitched squeal, your air conditioner may be trying to tell you something through the noises it makes. Some noises are nothing to worry about, while others are signs of serious (and potentially expensive) problems. Here's an easy guide to help you listen for normal and abnormal in the realm of air conditioner noise.

Normal air conditioner noises

Air conditioning units aren't always silent, and noises aren't always a bad thing. The following noises are normal to hear before, during, and after your air conditioner turns on and cools your home.

Startup "takeoff"

It might sound like a tiny airplane is taking off when your air conditioner starts up. This is the result of many moving parts coming together and starting up at once. The noise should go away after a few seconds, but may return anytime a new cooling cycle starts.

Blowing

A low, steady blowing noise is normal for an air conditioner, as it continually blows cool air through the ducts and vents of your home. This noise should be quiet enough to disappear into the background. If the noise more closely resembles buzzing or humming than a gentle blow, loose parts or electrical problems may be to blame.

Clicks

You'll likely hear your air conditioner make a series of clicking noises after completing a cooling cycle. This is normal. What you're hearing is the machinery coming to a stop. A constant clicking noise, however, can be a sign of a failing thermostat or an object stuck in the fan assembly.

Air conditioner is too loud

Normal noises may not indicate problems, but that doesn't mean they can't get annoying. Your neighbors may also not enjoy loud sounds coming from the outdoor unit. If you want your air conditioner to quiet down, you have several options. First, upgrading to a newer, more efficient, and quieter air conditioner may be the way to go if your unit is several years old. Modern A/C units take quiet comfort to the next level. If you're content with your current system, you can have an HVAC professional install a quiet fence or sound blanket to muffle the unpleasant noises. A quiet fence is an enclosure that, when installed around an HVAC unit, physically blocks noise. Sound blankets are constructed with multiple insulating fabric layers, allowing them to absorb noise.

Abnormal air conditioner noises

When all is going well, you probably won't hear very much from your air conditioner. But, when things start to go wrong, new noises can pop up. If you hear any of the following noises during the normal operation of your air conditioner, call a professional to accurately diagnose the problem.

Banging

Banging noises are usually a sign that an internal party has come loose. It could be that a screw, spring, or bolt needs to be tightened. Or, a component may have shifted out of place. In either case, this problem should be addressed immediately. Turn your air conditioner off to prevent further damage before calling for service.

Buzzing

Motor problems and capacitor issues are two of the most common causes of buzzing noises. They can also indicate:

  • Loose parts
  • Debris in the indoor or outdoor unit
  • Loose or out of place fan blades
  • A dirty condenser coil
  • Electrical problems

Hissing

A hissing air conditioning noise that only lasts for a few seconds is probably not a problem—it's likely just the expansion valve, or your ductwork. However, if the noise persists, it can be a sign of the following:

Refrigerant leaks

Refrigerant is stored in copper tubing throughout the AC system. Leaks in these copper tubes allow the high-pressure refrigerant to escape, creating a hissing noise.

Internal valve leaks

The internal valve in the compressor can produce a hissing noise when it leaks. These leaks should be repaired as soon as possible, since without these valves, refrigerant gas will not be pressurized correctly.

Compressor problems

High-pressure conditions can cause the compressor to malfunction and hiss in the process. Replacing a compressor is very expensive, and it generally makes more financial sense to replace the air conditioner altogether.

Whistling

Whistling is often caused by low airflow. The most common culprits of low airflow are blocked air vents or dirty air filters. Whistling can also be caused by a loose or damaged seal somewhere in the unit. Luckily, both of these are easily fixed.

Bubbling or gurgling

Bubbling or gurgling sounds usually mean that your compressor has a refrigerant leak. Refrigerant leaks can cause a whole host of cooling problems, and should be addressed as soon as possible.

Screeching, screaming, or squealing

A shrill, sharp, and unpleasant screeching or squealing noise can indicate several possible problems. Components like the blower wheel and fan motor will make this sort of noise when they're in need of replacement. Sometimes, adding lubricant can help quiet down a noisy motor. Screeching can also be a sign of a damaged belt or faulty bearings.

In rare cases, high-pitched squealing noises are caused by a dangerous internal pressure buildup, which should be considered an emergency situation. If you suspect this is the problem, turn off the air conditioner immediately and call for emergency service. While this is less than ideal during the hot summer months, you don't want to risk causing even more costly damage by running your air conditioner in this case.

Rattling

A rattling noise coming from the outdoor compressor and fan usually points to loose hardware or a failing motor. Generally, the louder the rattling, the more serious the problem. You can do your part to get to the bottom of the problem by checking for loose screws or bolts and tightening them if any are found. Ratting can also result from twigs, leaves, and other debris clogging the outdoor unit.

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