Noisy Split AC and Bad Smells: Causes and Solutions
Solusi Karya Nusantara Technical Team · 2026-10-19 · 5 min read
A split AC that normally runs quietly but suddenly turns noisy, or one that gives off a bad smell when switched on, often worries homeowners. Fortunately, the sounds and smells that appear usually follow certain patterns that can help point to the cause. This article covers the most common types of noise and odor, their likely causes, and the safe steps you can take before calling a technician.
Why Does a Split AC Get Noisy or Smelly?
Under normal conditions, a split AC only produces a soft hiss from the airflow. Once a component gets dirty, loose, or damaged, the sound and smell it produces change. Noise usually comes from mechanical parts (the fan, motor, bracket) or the refrigerant flow, while odors typically come from dirt, water, or — in a case worth watching closely — overheating electrical components.
Common Types of Noise
1. Rattling Noise
A noise like a loose object vibrating usually comes from loose mounting bolts, a cover panel that isn't fitted tightly, or a small object (a twig, dry leaves) that has gotten into the outdoor unit's fins.
How to fix: Check whether the indoor/outdoor unit covers are fitted tightly. For the outdoor unit, check whether any foreign object is caught in the fan. If the source of the noise isn't obvious, leave the inspection to a technician rather than opening the unit yourself.
2. Squealing Noise
A continuous high-pitched squeal, especially right after the AC is switched on, often comes from a fan motor that is losing lubrication in its bearing, or a belt that has started to wear out on some outdoor unit models.
How to fix: This noise signals that a mechanical part is already wearing down and should be checked by a technician soon, before the fan motor fails completely and needs replacing.
3. Clicking Noise
A brief click when the AC is switched on or off is usually normal — it's the electrical relay engaging. However, repeated clicking that continues throughout operation can indicate an unbalanced fan or a faulty electronic component.
How to fix: If the clicking only happens briefly at startup or shutdown, there's no need to worry. If the clicking continues throughout operation, note the pattern and mention it to the technician during service.
4. Hissing or Bubbling Noise
A faint hiss from the indoor unit can simply be the normal sound of refrigerant flow. However, a fairly loud hiss combined with weaker cooling than usual can indicate a small leak at a refrigerant pipe connection.
How to fix: Check whether the AC also feels less cold than usual. If so, don't add refrigerant or open the pipe connections yourself — this is work for a certified technician with the right pressure-measuring tools.
5. Compressor Hum from the Outdoor Unit
A low, constant hum from the outdoor unit is simply the normal sound of the compressor working. However, a hum that suddenly becomes louder than usual, especially combined with excessive vibration at the unit's mounting, can indicate the compressor is starting to fail or that the mounting is no longer solid.
How to fix: Check whether the outdoor unit's bracket or feet are still tight and level. If the hum keeps getting louder over time, schedule a technician inspection soon.
Common Types of Smell
1. Musty/Moldy Smell
This is the most commonly reported smell, usually strongest when the AC is switched on after sitting idle for a while. It's caused by mold and mildew growing on the evaporator, filter, or drain pan when these stay damp and aren't cleaned regularly.
How to fix: Wash the air filter and schedule regular cleaning of the indoor unit. We've covered the full guide to this in our article How Often Should a Split AC Be Cleaned?.
2. Burning Smell
A smell like burning plastic or wiring is a warning sign that should never be ignored. It can be caused by an electronic component short-circuiting, a failed capacitor, or frayed wiring touching another component.
Safety note: If you smell burning or scorching, immediately switch off the AC at the breaker and unplug it, then call a technician. Do not turn the AC back on until it has been inspected, since a short circuit can lead to a fire.
3. Rotten Egg/Sewer-like Smell
A strong sewer-like or rotten-egg smell usually doesn't come from inside the AC itself, but from the drain line connected to the home's wastewater pipe, especially if there is no water trap on the drain path.
How to fix: Check whether the drain hose connects directly to the wastewater line without an air gap. Adjusting the drain path is best left to a technician so the smell doesn't rise back into the room.
4. Cigarette or Food Odor
An AC running in a room with cigarette smoke or strong cooking smells can absorb those odors into the filter and evaporator, then blow them back out every time it's switched on even after the source of the smell is gone.
How to fix: Wash the filter more often if the room is frequently exposed to cigarette smoke or strong cooking smells. Periodic evaporator cleaning also helps remove odors that have already built up.
When to Call a Technician
Call a technician right away if you smell burning (switch off the AC first), hear a squeal or a hum that keeps getting louder over time, suspect a refrigerant leak from a hissing sound, or if a musty smell persists after you've cleaned the filter and unit yourself. Do not open the unit, touch electrical components, or open refrigerant pipe connections without the right tools and expertise.
Solusi Karya Nusantara provides AC cleaning, leak repair, and electrical inspection for split AC units for homes and businesses across Medan and North Sumatra, with Daikin-certified technicians.