Unusual noises coming from the furnace while in use can be caused by a variety of issues. Some issues are simple and can be taken care of easily, while others are more serious and will require the expertise of an HVAC specialist. Please note that attempting to fix any of the issues described in this article can be potentially dangerous, and you should always consult with an HVAC professional.
Squealing or Whistling Noises
Whistling noises coming from your furnace might be caused by gaps or holes in the ductwork. These gaps and holes are likely located at the connections between the duct and the furnace.
Another possible cause of the whistling noises is a clogged filter. If the HVAC filter is clogged, the fan will create increased pressure in the duct which will cause air to be sucked from even the tiniest holes or gaps in the ductwork. Replace your dirty filter, this will likely reduce the whistling noise and reduce the stress on the HVAC fan motor.
Whistling noises can be emitted from your HVAC system if ducts are too small.
Squealing noises from the blower motor can be caused by worn belt fan or bearings being worn out. If you suspect a belt or worn bearings are the cause of the noise, contact an HVAC specialist as soon as possible before the motor breaks down.
Ductwork Rattling or Banging Noise
Rattling in the furnace or ducts is usually caused by vibrations in the loose ducts or if the furnace substrate is loose or not level. This can be fixed by adding screws or duct tape to any loose pieces or parts of the ductwork. If the furnace substrate is loose or not level, it may be necessary to add rubber or cork pads under the furnace.
If you hear banging in the ducts as your furnace starts, “oil-canning” or flexing of the ductwork due to the pressure changes might be an issue. Flexing occurs when ductwork has a weak spot. This can be fixed by adding a thicker piece of metal across the weak spot in the ductwork.
Furnace Motor Noise and Banging
The noise coming from the furnace motor may be due to what is called “bouncing” or due to worn bearings.
If a loud noise is coming from an oil fired furnace as it starts, this could be caused by a serious issue called “delayed ignition.” This issue occurs when unburned oil builds up in the fire box and suddenly ignites all at once. This is dangerous, and a qualified HVAC specialist should be contacted immediately.
Another possible cause of a banging noise coming from a gas-fired furnace might be an “ignition roll-out.” This potentially dangerous problem is caused by a little explosion at the furnace start-up and it is usually followed up with a burst of flames. If you suspect a gas ignition problem, you should watch the furnace start up with the furnace door open so you can see flames. If you see additional flames, or shaking accompanying with a banging noise, you have potentially identified a very dangerous problem. Please call an HVAC specialist immediately. Newer furnaces have a “roll-out sensor” installed that will shut down the furnace if this problem occurs.