Updated January 16, 2024

Ductless mini-splits are becoming increasingly popular as they offer an energy-efficient and effective way to heat and cool any rooms or buildings without central HVAC. They are also a great way to supplement your existing heating and cooling should you have any rooms that constantly feel too warm or too cold. As with any other HVAC unit, mini-splits can suffer from a variety of different issues that can affect their performance and cause them to suddenly be much louder or make unusual noises. Let’s look at the most common noises your mini-split system might make as well as what issues can cause them and how they can be overcome.

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1. Cracking or Popping

Hearing cracking or popping noises coming from your mini-split indoor air handler unit is extremely common and usually nothing you need to worry about. Whenever your mini-split system is cooling, the plastic housing on the air handler contracts as it cools and then expands again as it begins to warm up once the unit shuts off. The opposite happens when heating as the plastic expands when the unit is on and then starts contracting after it shuts off.

This cycle of expansion and contraction is quite minor and shouldn’t ever cause any damage to the unit. However, the plastic will sometimes make audible popping or cracking sounds as it contracts and expands. Unfortunately, there really isn’t much you can do to prevent or dampen this sound, but luckily, it usually isn’t loud or frequent enough to be more than a minor annoyance.

2. Rattling

There are also times when you may hear a rattling sound coming from your mini-split system. This noise most commonly comes from the outdoor condenser unit, but the indoor air handler may also rattle. In either case, this sound almost always indicates that either something inside the unit has come loose or a foreign object has gotten inside.

If your air handler is rattling, the first thing you should do is check to make sure the cover is snapped in place securely. If the cover isn’t tight, the vibrations from the fan will often cause it to rattle whenever the unit is running. If the unit still rattles after you’ve checked the cover, you will want to have it inspected so that the loose component can be found and tightened.

Rattling noises coming from the outdoor condenser could indicate that the fan assembly, cover, or some other component isn’t fully secured. Leaves, twigs, and other debris can also get inside the unit and cause it to make a rattling sound. In either case, you will need to have an HVAC technician inspect the unit as the only way to access the inside is to remove the fan assembly. It definitely isn’t recommended to do this on your own as you could easily cause serious damage if you don’t know what you’re doing.

3. Gurgling or Swooshing

If you use your mini-split system for heating in the winter, it will occasionally need to switch over to defrost mode to prevent the coils from icing up. In freezing weather, a mini-split unit will typically need to defrost once every hour or two.

When a mini-split system heats, it uses cold refrigerant to absorb heat from the air outside. This process partially heats the refrigerant, which then flows through an expansion valve so that it instantly becomes extremely hot. Finally, the hot refrigerant flows into the evaporator coil, and the heat is released from the refrigerant into the air.

When defrosting, a mini-split system reverses the direction that the refrigerant flows and switches over to cooling mode for a short time. This means that hot refrigerant then flows through the coils in the outdoor unit to help melt any ice that may have started to form and ensure the unit doesn’t completely freeze up. As the unit defrosts, you will occasionally hear gurgling or whooshing sounds coming from the outdoor compressor.

This is another sound that you typically don’t need to worry about. The only time you do really need to worry is if it seems like your mini-split unit is constantly defrosting as this is a sign that the unit isn’t working as it should. This problem most commonly happens because the refrigerant level is low, but it can also occur due to a malfunctioning valve or sensor or because the control board has gone bad.

4. Humming or Buzzing

Humming and buzzing noises coming from the outdoor mini-split unit are potentially more serious and usually indicate some electrical issues. These sounds most commonly occur due to a loose electrical connection or some other problem that is causing the unit to draw more current than normal and thus overloading it. Unfortunately, electrical issues aren’t always easy to diagnose and will definitely require having the system professionally inspected. This is something that you don’t want to put off as it could lead to the unit overheating or suffering other major issues.

5. High-Pitched Whistling or Hissing

A loud hissing or whistling noise coming from either the indoor or outdoor mini-split unit is another problem that you definitely shouldn’t ignore. These noises almost always indicate that there is a refrigerant leak somewhere in the system. Due to the pressure of the refrigerant, it will usually produce a fairly loud whistling or hissing noise as it escapes.

If you’re lucky, it may be that one of the refrigerant lines that run between the indoor and outdoor units has a small hole or that one of the valves is damaged or loose as these issues are fairly easy to repair. A much more serious problem is if the coils in either of the units are leaking as this isn’t always possible to repair.

A mini-split unit requires a very specific level of refrigerant charge in order to work effectively and efficiently. If the refrigerant is low due to a leak, your mini-split system won’t ever provide as much heating and cooling. Still, the bigger issue is that not enough refrigerant in the system puts a huge amount of added stress on the compressor and could lead to the motor overheating and burning out. A leak can also allow liquid refrigerant to flow into the compressor, and this can ruin it almost instantly.

This is the absolute last thing you want to happen as it typically means you will need to have your entire mini-split unit replaced. As such, we always recommend that you immediately shut off your system if you ever hear any hissing or whistling sounds and avoid using it again until you can have a technician inspect it and check the refrigerant charge.

Ronkonkoma’s Heating and Cooling Experts

If your mini-split system is making unusual noises or has any other issues, the certified technicians at Cool Power LLC can quickly determine what the problem is and how best to fix it. Located in Ronkonkoma, NY, we service and repair all makes and models of mini-split units. If you’re considering a new system for your home, we can also help you choose which unit is the best fit and handle the entire installation process. We also install and service central HVAC units, including air conditioners and furnaces. Give us a call today if you need to have your mini-split unit inspected or to schedule any other heating or cooling service.

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