Noise

Noise

NOISE – An air conditioning system starts making an unusual noise.  The noise may be generated from the indoor or the outdoor unit.

NOISES FROM THE INDOOR UNIT – Typically, noises generated from the indoor unit are caused by some type of problem in the blower system.

Loose Belt

  • Some older furnaces use a belt-drive motor and blower system.  When the belt becomes loose it can generate a squeaking or squealing (especially on start-up). A damaged belt may cause a thumping noise.

 Bad Blower Bearings - Qualified Technician Recommended

  • Belt-drive blower systems will also have external bearings at each end of the blower shaft.  A bad bearing can generate noises that range from a high-pitch squealing noise to a low-frequency thumping sound.  If you suspect bad blower bearings, you should contact a qualified technician to diagnose.

 Broken or Loose Blower WheelQualified Technician Recommended

  • A common symptom with blower wheels is a noise I can only explain as a loud “V-V-V-V-V-V-V”.  This is caused by blower wheel blades that have broken their weld connections and vibrate as the wheel turns.  It is possible to continue operating the blower in this condition, but action should be taken to repair as soon as possible.
  • A broken blower wheel, on the other hand, may result in a loud clanging or banging noise from the furnace when the blower motor starts.  It may also cause the furnace to shake when running.  If you experience these symptoms the system should be turned “Off” and a qualified technician should diagnose.

Bad Blower Motor - Qualified Technician Recommended

  • If the blower motor does not start (no air blowing out of the registers) on a call for cooling, it can be the result of a faulty fan relay or the furnace control board, but often the problem is a bad blower motor.
  • If the blower motor hums but does not start, it is most likely a bad motor or a bad capacitor (below).

Bad Capacitor - Qualified Technician Recommended

  • Most direct-drive PSC fan motors require a capacitor which provides extra power for starting.
  • A fan motor with a bad capacitor will typically attempt to start but will fail while making a louder-than-normal humming noise.
  • After several seconds of humming, the motor will typically overheat and shut-off and then retry after about 30-60 seconds of cooling.
  • A fan motor that runs normally after helping to start by turning by hand usually indicates a bad capacitor. 

Air Leakage

  • Small air leaks in the furnace or ductwork (especially on the return air side) can cause a high-pitch whistling sound.  
  • A common source is around the air filter if the filter slot is open.  
  • Covering the air leaks with duct tape will generally eliminate these noises.

 Restricted Airflow

  • If airflow becomes blocked or restricted, the furnace blower can be put under greater strain and make noises that are louder than normal.  Look for a clogged air filter or closed/blocked registers.

NOISES FROM THE OUTDOOR UNIT – Typically, noises generated from the outdoor unit are caused by vibrations or metal objects coming in contact with each other. 

Fan blade out of balance 

  • Metal fan blades can become out-of-balance and make the outdoor unit shake while running.  This shaking can cause a wide variety of vibration-related noises.  
  • The unit can continue to run in this condition, but prolonged use may cause damage to critical system parts.  Steps should be taken to repair as quickly as possible.

Object in the fan path 

  • It is not unusual for foreign objects (sticks, weeds, etc.) to get through the fan screen and get in the path of the fan blade.  This will cause a loud clanging sound when the unit is operating. * Do not attempt to remove anything from the fan guard while the unit is operating.

Loose screws 

  • A common source of noise from the outdoor unit is loose panels rattling.  If you can make the noise from the outdoor unit go away by putting pressure on some part of the unit, there is likely a panel or component that is not fastened securely.

 Bad Fan Motor - Qualified Technician Recommended

  • If the outdoor fan motor does not start when the compressor comes on, it will cause very high operating pressures and a noisy compressor.
  • If the fan motor hums but does not start, it is most likely a bad motor or a bad capacitor (below).

Bad Capacitor - Qualified Technician Recommended

  • Most direct-drive PSC fan motors require a capacitor which provides extra power for starting.
  • A fan motor with a bad capacitor will typically attempt to start but will fail while making a louder-than-normal humming noise.
  • After several seconds of humming, the motor will typically overheat and shut-off and then retry after about 30-60 seconds of cooling.
  • A fan motor that runs normally after helping to start by turning by hand usually indicates a bad capacitor. 

 

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