A quiet hum from the outdoor unit is normal, that is the compressor doing its job. The problem starts when the hum turns into a vibration you can feel inside the house, or when metallic sounds appear that were not there before. Most of these noises have straightforward causes and can be fixed without calling anyone out.
What the sound tells you
The type of noise already points to the cause. A steady, constant hum is the compressor running normally. Vibrations that rise and fall with the load usually trace back to the bracket or the anti-vibration mounts. A hiss or whistle can mean refrigerant is escaping. A scraping or clicking noise is almost always the fan blade touching something. Identifying the sound correctly means you start in the right place instead of dismantling everything at random.
The most common causes
- Loose wall bracket. Over time the bolts fixing the unit to the wall work loose and the whole assembly vibrates. This is the number one cause of vibration felt inside the building.
- Missing or hardened anti-vibration mounts. Rubber pads should sit between the unit and the bracket. If they are missing or have hardened, vibration travels straight into the wall.
- Dirty or unbalanced fan blade. Leaves, dust and nest material make the fan heavier on one side and cause it to wobble.
- Compressor under stress. A deeper, louder-than-usual rumble can indicate a compressor struggling with low refrigerant charge.
- Vibrating casing panels. A missing screw on the outer casing lets the sheet metal panels rattle against each other.
What you can check yourself
Always isolate the power before touching the unit. Then start with the simple things.
- Tighten the bracket bolts. A wrench and a firm turn on each bolt fixes most vibration complaints. Also check the wall anchors holding the bracket itself.
- Clean the grille and fan. Remove leaves and debris. In autumn all sorts of things find their way in.
- Inspect the anti-vibration mounts. If they are flattened or missing, replacements cost a few euros and often transform the situation.
When to call a technician
If the noise is metallic, comes from the compressor, or is accompanied by reduced cooling output, switch the unit off and call a qualified engineer. A compressor running low on refrigerant can fail completely, and it is the most expensive component in the system. A worn fan bearing also needs the right parts and proper skills to replace correctly.
How to prevent the noise returning
Cleaning the outdoor unit at the start and end of each season keeps the fan clear and quiet. An annual check on the bracket bolts stops vibration building up again. If you are installing a new unit now, ask for it to be mounted on good-quality anti-vibration pads and positioned away from bedroom windows and neighbours' windows. Placement matters more than most people realise. Quiet models rated below 50 decibels are listed in our air conditioners section.