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My Outdoor Unit Makes Loud / Strange Noises

Outside AC Unit Making Loud Noise

Heat pumps do make strange loud noises especially in the wintertime. It's just the nature of the beast. i.e., heat pumps have reversing valves that reverse (or shifts) the flow of refrigeration between the heating and cooling modes. This occurs every time the unit goes into the "defrost cycle".

When this shifting occurs, there is a loud "swooshing" sound, which usually lasts for several seconds. The "swooshing" sound is sometimes followed by a somewhat louder than normal compressor noise as it shuts down to allow the refrigerant pressures to equalize. All these noises are perfectly normal.

Another common sound is present whenever the outside condenser turns "off" or "on". For a few seconds it sounds like an out of balance washing machine.

Sometimes customers note a "buzzing" sound from the outdoor unit. even when it's not running. This is usually the reversing valve's solenoid coil. It's low voltage (only 24 volts) so it's really not wasting energy.

If you hear a metal to metal sound or a persistent "thumping" sound, check the fan blade of the compressor to see if the blade can be hitting something (ice, wire, twig). Shut the unit off, immediately as this not only will ruin the fan blade by throwing it "out of balance" but if allowed to persist, ruin the fan motor as well.

And there is always "vibration" noise. Sometimes its only a matter of installing (or replacing) isolation pads. Sometimes the refrigerant lines are strapped too tight. Sometimes it is the unit itself and cannot be eliminated.

Below is a list of possible causes. Items in blue can be addressed or fixed by the homeowner. Those in red usually require a service call.

  • Outdoor unit iced up-weather related

  • Fan blades hitting some obstruction

  • Vibration due to out of balanced fan blade

  • Vibration due to loose parts

  • Vibration due to piping strapped too tight

  • Low refrigerant charge (gurgling noise)

  • Defective reversing valve-makes hissing sound

  • Defective compressor valve

  • Defective motor

  • Out of balanced or bent fan blades

  • Buzzing contactor or solenoid coil

  • Loud compressor

  • Loud unit

Hope this has been of some assistance, however they are just rough guidelines and not all possible conditions are covered.


The company You Choose To Install and Service Your System Will Determine The Degree of Comfort and Satisfaction You Enjoy.

Excerpts of the preceding information has been compiled by Hannabery HVAC, which has graciously permitted us to include them on this web site as a service for all homeowners.

These "Common HVAC Problems" are provided FREE and as a consequence Climatemakers does not assume any liability resulting from any information we provide. In all cases where there is any possibility of injury, consequential damage, direct damage to your equipment or when recommended by the equipment manufacturer, your system should be repaired, inspected or maintained by a qualified technician.

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