We’ve all experienced it: you turn your AC on and, even if it’s just for a moment, you hear that little clanking noise.

Once your system really revs up and starts to cool down your rooms, you might not hear or notice the noise anymore, but you know you heard it.

Most people ignore these noises and are then surprised when their system needs a repair. You can prevent big, costly repairs by listening to the noises that your AC system makes and diagnosing them. Here are some of the most common noises and what they mean:

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1. Clicking

If you are hearing clicking, this is probably an indication that the electrical components of your AC system are starting up. This clicking should be short-lived. If you continue to hearing clicking even long after your system is running, this is atypical and should be investigated. Clicking can mean that the temperature control in your thermostat is defective, which could make it very difficult to get and keep your rooms at a comfortable temperature.

2. Banging

This can be startling the first time you hear it, but if you hear banging in your system constantly, you might start to ignore it. Banging, however, is a telltale sign that there is a moving part in your system that is loose or broken. Most commonly, a blower, crankshaft, or piston pin has come loose. This might not cause your entire system not to work, but it can definitely impede it and lead to more serious issues down the road.

3. Buzzing

If you’re hearing a buzzing noise come through your system, there are a number of different things that might be causing it. Loose parts is the most common, but it could also be a fan that is about to fail. Your fan blades might be out of balance, your condenser might need to be cleaned, you need to clean your filter, your blower is bad, or the refrigerant is leaking.

4. Humming

Your system will probably hum a little bit—every AC system, even the quietest ones, will make a little bit of noise as they run. However, if you are hearing a humming that is out of the ordinary, this could be caused by loose parts or piping that is being overtaxed. Loud humming, especially if it is accompanied by buzzing, is a good indication that there is about to be something seriously wrong with your AC.

5. Screaming

Screaming is a really good indication that your system needs to be looked at by a professional immediately. High-pitched whistling and screaming is likely caused by a leak of the refrigerant. This can seriously damage your system is not immediately corrected and it can also be extremely dangerous for any people who might be breathing the air that is being cooled by that faulty system. It also might mean that there is high pressure building up in your compressor, which, again, is not good.

6. Rattling

Rattling probably means that several of the parts in your air conditioner are loose. This is, again, another instance in which you should probably turn off your air conditioner and call a professional HVAC service to look at it. A lot of rattling means that your system is deteriorating. A professional will be able to tell you whether or not it just needs to be tuned up or if you might need to get an entirely new system.

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