Common Causes:

1) An unbalanced Condenser Fan blower.

a) This is a very rare condition, but I’ve personally come across such before. Normally, the outdoor condenser fan has 3 fan blades. Should any of the fan blade breaks off, the fan will be so unbalanced in weight that it causes the entire condenser to vibrate vigorously. If you suspect this, try to see if any of the fan blades had broken off.


for your info** 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:

The fan blade will need to be replaced. However, most manufacturers build the fan in such a way that it is a perfect fit with it’s fan motor. As such, using another brand’s fan blade wouldn’t be economical as it involves modification. Will need to buy the original part from the manufacturer and pray that your model is not obsolete.

2) Worn out compressor shock absorbers.

There are usually 3 pieces of rubber shock absorbers placed underneath the compressor. Their function is to cushion the vibration of the compressor for noise reduction purpose. Since the compressor can be very hot; after some time, the rubber may hardened or melt (if made from inferior quality). If it hardened, the shock absorbing properties are poor and the condenser becomes noisy. If it melts, the base of the compressor touches the base of the condenser, metal knocking against metal, it’ll be very loud. Not an expensive item, but very labour intensive to replace. This item is not brand dependant.

Solution: Replace the worn out shock absorbers. $ 250 per compressor

3) Faulty Condenser 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 and burn the copper windings, it will be too late for bearing replacement, the motor is out.

Solution: Replace the faulty fan motor bearings, cost $250

4) Jammed Condenser Fan motor bearings.  

Another common cause of noise is caused by a jammed condenser motor bearing. Normally, this will be the symptom:

1) The aircon will only be cold for 1-2 mins.
2) Subsequently, the outdoor condenser gets rather loud and after some time, the noise will stop and the aircon is no longer cold.

This is because when the condenser fan motor jams, it will not be able to cool the condenser/compressor. For the first 1-2 minutes, it is still ok, but subsequently, the pressure inside the compressor will build up. As the pressure builds up, the compressor vibrates vigorously, this is the noise you are hearing. Heated above 150’C, an overheating relay placed above the compressor ill be activated, shutting the compressor down. So the noise stops and of course, the aircon is no longer cold.

Solution: Replace the faulty fan motor bearings, cost $250


5) 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. 6) Acoustic effect caused by hollow panel separating condenser and room.

This is one of the most common source of noise. But it is often a combination of a faulty shock absorber as mention above in “1” and the hollow space created by panels separating the condenser and the room. What happens here is that the condenser is hung outside of the window on stainless steel brackets. In the olden days when air conditioners are not 
split units, they are what we called “window air conditioners”. HDB flats were built with a part of the wall made of wood so that when an apartment owner wants to install the “window air conditioner”, the wooden panel can easily be removed rather than to dangerously hack the wall and risk falling bricks from the high rise apartment.

Now that the condenser is hung outside of the wall, in front of the said panel. To beautify the inside of the room, the wooden panel is often removed and the area sealed off using plaster wall. The outside needs to be water proofed, so usually aluminium sheets are used to cover the exterior wall. But this creates a hollow between the indoor plaster wall and 
the said aluminium sheet. This empty space like a piano or guitar, amplifies the noise produced by the condenser.

Solution: Stuff some foam, sponge or pillow to fill up the empty space and the noise will be dramatically reduced.

7) A jammed compressor.

If you hear a loud rattling sound that comes from the condenser for not more than 2 seconds, takes a break for a few seconds and then rattles again, and then takes a long break and on and on. Then what that you are hearing is the sound of a jammed compressor.

When the compressor bearings are jammed, obviously the compressor motor shaft can’t turn, but the magnetised coil causes the motor shaft to vibrate, this is the source of the loud vibration. When this happens, it will draw an abnormally high current which will threaten to trip your circuit breaker. To prevent said tripping from occuring, compressors are fitted with a thermal/high current breaker. When your compressor overheats or draws abnormally high current, this breaker will open.

If the compressor breaker is activated by high current, it will momentarily stop the rattling sound as no more current will pass through the compressor. However, since no more current is passing through and obviously no more high current, the breaker will once again close. Normally it takes a few seconds. This explains the few seconds of silence followed by another round of rattling.

However, if the breaker is activated by overheating of the compressor, it will only spring back after the compressor loses heat, which will normally take 1-2 hours.