Mini Split Making Noise: Which Ones Are Normal

Rule out the normal list first

Gurgling, hissing or a sound like water trickling. Refrigerant moving inside the head. Loudest in the first few minutes (Fujitsu says 2 to 3), again when it shuts down, and it changes note as the unit speeds up and slows down. Every brand does it.

Clicking as it starts and as it stops. The little motor driving the louvre open and shut. A handful of clicks, then quiet.

Ticking and creaking for a few minutes after it stops. Plastic shrinking as it cools, the same thing a kettle does. It comes from the case rather than from anything turning.

A clunk and a whoosh outside, then steam, in cold weather. That is the defrost cycle turning the system round to melt the outdoor coil, and it belongs to winter heating: frost that clears against ice that does not.

A hum outside that builds for a few seconds and then settles. An inverter compressor ramps up instead of banging on, which is why a mini split sounds nothing like the air conditioner next door.

Rattle, whine, or water

A rattle that started after somebody cleaned the filters. This is a common one. The front flap lifts and the two mesh filters slide out, and a filter that is not seated, or a flap clipped at one end only, buzzes against the case every time the fan runs. Open it, take both filters right out, then put them back so they lie flat and the flap clicks shut at both corners.

A whine or a whistle that changes pitch with the fan speed. That is the fan itself. It is a long thin barrel with dozens of small blades, so one leaf, one bit of paper or one dead insect unbalances the whole thing. Power off at the isolator, then shine a torch up past the louvre and look along it.

A whine that stays at the same pitch whatever the fan does. Electronics rather than the fan. Some inverter boards sing quietly at certain output levels, and a faint one that has always been there is how yours sounds.

A squeal that is there from the moment the fan starts. A dry bearing in that fan. It gets worse week by week and it will not recover on its own.

Water running, sloshing or dripping inside the head. The condensate drain rather than the machine. A slight trickle is normal, sloshing means the pan is holding water it should be losing, and the thing that follows sloshing is water down your wall: how to clear the drain line.

A buzz that stops when you press a hand on the case. The head is not pulled tight to its mounting plate, or a screw behind it has worked loose.

Start with the bracket, not the machine

A hum you hear more clearly indoors than outdoors. Look at how it is mounted. A unit hanging on a wall bracket is bolted to the house, so the ordinary hum of a compressor goes into the brickwork and comes out again in whichever room shares that wall. Rubber anti vibration mounts between the feet and the bracket cut it, and they cost very little. A unit standing on the ground on pads rarely does this.

Rattling that stops the moment you rest a hand on a panel. Panel screws. They work loose from vibration over a couple of seasons, and there are usually more of them than you would think. Go round every one.

A rattle that rises and falls with the fan. Something in the fan guard. Leaves, twigs and the odd stone find their way in there, and a stone will mark the blades.

A knock or a tick where the pipes leave the wall. The two copper pipes expand and contract as the temperature changes, and they tap against whatever they are clipped to. Foam insulation or a rubber grommet at the wall settles it.

A loud hum with the fan not moving at all. That is the one from the warning box. The fan motor, or the capacitor beside it, and both of those mean somebody out: what a capacitor is and why it is not a homeowner part.