NOISE – An unusual sound is coming from the furnace.


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 noise (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

  • After the burners have ignited and proper ignition has been detected by the ignition control board, the next step in the heating cycle is starting of the blower motor.
  • This will be done either by a heat-activated switch or internal timing of a control board.  In either instance, the blower motor should start within about 1 minute after ignition.
  • If the blower motor does not start (no air blowing out of the registers), it can be the result of the activating switch 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. 

Delayed Ignition - Qualified Technician Recommended

  • The burners on a gas furnace should lite quickly and smoothly.  If one or more burners fails to lite properly, the excess gas build-up in the furnace may cause a "boom" noise when the burners ignite.  This condition is usually caused by dirty burners and should be diagnosed by a qualified technician.

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.


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