Common Causes:

1) An unbalanced aircon blower.

a) The circular aircon fan is too dirty, the weight of the dirt may not be evenly distributed and thus causing the aircon fan to vibrate when turning.

b) The balancing chips attached to the circular blower had rusted and dropped off, leaving the blower unbalanced. Or some blades were broken accidentally while you or your technician attempts to clean up the blower.

For your information only:

Any spinning round object must be perfectly balanced, if it gets heavy one sided, it will vibrate. The “vibrate” mode on your mobile phone is made by attaching a semi-circle weight (thus an extremely unevenly distributed weight) to a miniature motor.

Solution:

Properly cleaning up the circular blower using chemical and high pressure water jet (using our method). If it is still vibrating, weight must be added to balance the blower. Aircon Servicing.

1b) A Hardened Motor Cushion (Shock Absorber)

The reason why I’ve classified this problem under 1″b” is because this problem is closely linked to the problem illustrated above at “1”. The vibration caused by an unbalanced fan are normally absorbed or cushioned by the rubber paddings on the fan motor, this is a motor cushion:

Since a fan motor is hot, the rubber can get stiffened after some years. Depending on the manufacturer’s selection of material, certain brands of air conditioners are more prone to this problem. But typically, it takes more than 5 years before this rubber becomes hardened, some even deforms. Replacing this item will cost $168.

2) A Choke Aircon Fan Coil.

a) When your aircon fan coil is choked, the wind has difficulty coming out, the air in the blower compartment will go into a compression / decompression state. You will hear sounds like phh….phhphh…phhhhh…..phhphh.

Thus if you put you hands very near to the blower like this:

You will feel that wind is not coming out smoothly, some parts of the blower may not have any wind coming out.

Solution:
Hydro jet or ChemJET servicing

2) Faulty Aircon Fan motor bearings.  

Another common cause of noise is a screechy motor bearing. Replace them as soon as possible because they will jam soon. When the motor jams, it may overheat, damage the shock absorbers as mentioned above, or worst; burn the copper windings, it will be too late for bearing replacement, the motor is out. On top of that, when the motor jams, the fan coil will ice up as the “coldness” is not dissipated by the fan. This can cause:

1) Compressor failure as non evaporate liquefied gas returns directly to the compressor.

2) Short circuit to the electronics if ice gets into the PCB section.

3) Condensation to the copper pipes, dampening the insulators and causing staining on your walls and/or false ceiling.

3) Abnormal Gas Pressure

This applies to people with very sensitive ears. Having mentioned in my earlier write up under How Reliable is your compressor, it was mentioned that liquefied gas is evaporating inside the aircon fan coil copper pipe. For those with sensitive ears, you can hear the sound of the gas evaporating. When it evaporates too fast, the noise level is louder. It may be a sign of a low pressure environment. Great care must be taken before topping up of gas.

4) Manufacturer defect (aluminium fan coil problem)

Rare, but it does happen; the air conditioner produce a continuous rattling sound that is not caused by vibration nor fan blades rubbing against anything. The sound comes from the aluminium fan coil itself. As air passes through the defective aluminium fins, the fins rattles (you cannot see the vibration); producing a continuous rattling sound. Since replacement 
of the fan coil is often not a desirable option (when warranty is over); isolating the problem area can be considered. This can be as simple as using a piece of plastic to block air from blowing pass the problem area.