Quick answer
A musty smell from an AC usually comes from dirty filters, a damp coil or a drain tray with mould, and a proper cleaning service fixes it. Rattling, grinding or loud buzzing points to loose parts, a worn fan motor or a failing compressor part. A burning smell is serious: switch off the AC at the wall straight away and call a technician.
Signs you may notice
- Musty, damp or "wet socks" smell when the AC starts
- Burning or plastic smell from the indoor unit
- Rattling or vibrating sound from the outdoor unit
- Clicking, squealing or grinding noise from the fan
- Loud buzzing or humming while the compressor tries to start
- Hissing or gurgling sound near the pipes
Common causes (most likely first)
- 1
Dirty filters and damp coil (musty smell)
You can checkDust and moisture on the filters and coil allow mould and bacteria to grow. The smell is strongest when the AC is first switched on, especially in the monsoon.
- 2
Blocked drain tray with standing water
Needs technicianWater that does not drain sits in the tray and starts to smell. It needs proper cleaning of the tray and drain pipe.
- 3
Loose panels, screws or mounting
Needs technicianVibration over time loosens screws, stands and covers, causing rattling. The outdoor unit stand may also need tightening or rubber pads.
- 4
Worn fan motor or damaged blower
Needs technicianA dry fan motor bearing squeals or grinds. A cracked or dirty blower wheel can make a thumping sound as it spins.
- 5
Electrical fault (burning smell)
Needs technicianA burning smell can come from overheated wiring, a failing capacitor, or a damaged motor. It can be a fire risk and should never be ignored.
- 6
Refrigerant leak (hissing sound)
Needs technicianA continuous hissing near the pipes may be gas escaping from a leak. Cooling usually drops at the same time.
- 7
Debris in the outdoor unit
Needs technicianLeaves, plastic covers or bird nests inside the outdoor unit can hit the fan blades and make a loud noise.
Safe checks you can do yourself
- 1If you smell burning, switch off the AC and the wall switch immediately and do not restart it.
- 2For a musty smell, switch off the AC, wash the filters with water, and dry them fully before refitting.
- 3After using the AC, run it in Fan mode for 10–15 minutes before switching off to help dry the coil (if your model has this mode).
- 4Check around the outdoor unit for loose items, plastic sheets or leaves near the fan, and remove them only from the outside without opening any cover.
- 5Note when the noise happens (at start-up, all the time, only outdoor unit) to tell the technician.
Always switch off and unplug the appliance before checking. Never open panels, touch wiring or try to refill gas yourself.
When to call a technician
- Any burning, electrical or plastic smell
- Grinding, screeching or loud banging noises
- Musty smell that stays even after cleaning the filters
- Hissing sound together with poor cooling
- The outdoor unit shakes or vibrates heavily
FAQ
Frequently asked questions
Why does my AC smell bad when I turn it on?
Usually because of mould or dirt on the filters, coil or drain tray. Cleaning the filters helps, and a full service with coil and tray cleaning removes the smell properly.
Is a burning smell from the AC dangerous?
It can be. Switch off the AC at the wall and get it inspected, because it may be overheating wiring or a failing electrical part.
Why is my AC outdoor unit making a loud noise?
Common causes are loose screws or mounting, debris hitting the fan, a worn fan motor, or the compressor struggling to start. A technician can find which part is causing it.
Is some noise from an AC normal?
A soft hum, air sound and occasional clicks when it starts or stops are normal. Rattling, grinding, screeching or loud banging are not.