Loud furnace noises come from all kinds of reasons. The problem is to understand the cause, and from there, see how the situation can be remedied. You may be able to fix some of the problems yourself; others will require the expertise of an HVAC specialist.
Loud noises coming from your furnace while it is running could be caused by any of a number of issues. For example, you may have a problem with the fan that blows hot air through your vents. That might be due to the fan bearings being worn out. In that case, these bearings will need to be replaced.
If your furnace is making a loud noise just as it starts, this could be caused by a serious problem called “delayed ignition.” This occurs when unburned oil builds up in the firebox chamber and is ignited all at once. This can be dangerous, and you should call a qualified HVAC specialist immediately.
If your furnace is whistling, one possible cause is that there may be gaps in the duct. You should try to hear exactly where the whistle is coming from. Often, it will be from a small hole near the connecting point between the duct and the furnace close to the blower.
Especially if the whistling has started recently, you may have a clogged filter, causing the fan to suck air from anywhere it can. Even tiny holes or spaces will generate a whistle if any significant air is being sucked through them. To test out this possibility, pull the filter out and see if the whistling stops.
If your filter is so caked with dirt that it is clogged and stopping the airflow, you will be better off taking the filter out (while arranging to purchase a new one). This is better not just to reduce the noise, but also to reduce the wear and tear on your fan motor.
Another possible cause for whistling would be that the ducts are too small. The furnace may need greater airflow and thus larger ducts.
Rattling and Vibration
If there’s rattling in your furnace and ducts, such vibrations may have simple fixes. Here are a few things you can do to try to stop the rattling:
- Add more screws to any loose ducts
- Apply duct tape to any loose ducts or other pieces of the equipment
- Add rubber or cork pads under the furnace. (You can likely raise the unit with a pry bar just enough to slip something thin underneath.)
If you believe the noise is coming from the furnace’s motor, this may be due to “bouncing.” You may be able to alleviate this bouncing by placing a sturdy piece of Styrofoam under the squirrel cage. However, if the bearings are worn out, you may need to call in an HVAC specialist.
Banging When the Furnace Turns On
If you hear banging when your furnace turns on, there are two likely issues. One is that your ductwork is “oil-canning,” or flexing when the pressure changes. Or it could be a small “explosion” called an “ignition roll-out.”
If the issue is oil-canning ductwork, you may be able to fix this. You should listen carefully to the furnace to see if the banging is coming from somewhere in the duct. If oil-canning is the problem, there is probably a weak spot in the ducts that needs support. You may be able to fix this by screwing a slightly thicker piece of metal across and onto the weak spot.
The other possible cause – ignition roll-out — is not very common, but it can be alarming. It’s a little explosion when the furnace starts up, or a burst of flame bigger than it needs to be.
To see if the banging is caused by a gas-ignition problem, watch the furnace start-up. Leave the doors on the furnace. If you can see into the furnace doors, you should be able to see what is happening. If not, listen in order to pinpoint where the noise is coming from. Watch to see if the doors on the furnace are at all shaking. If they are, then try again with the door to the burner compartment open, to see if the banging still occurs and if you can see what happens when it does.
If you see extra flame or shaking associated with your noise, you have identified a potentially dangerous problem and can now correct it. You should call in an HVAC specialist. Trying to deal with the gas portion of your furnace should be left to professionals. You should note that newer furnaces have a “roll-out sensor” that will shut down the unit if this problem occurs.
Banging or Squeaking When You Walk on the Floor
If you hear banging or squeaking when you walk on the floor near the furnace, the duct may be too tightly secured, or the metal panning may not have been nailed up correctly. This may be alleviated either by:
- Reinforcing the duct, as with an oil-canning duct
- Re-hanging the duct
- Replacing it with “Thermo-Pan,” a high-quality sheet metal alternative for constructing return air ducts between. This product consists of corrugated insulation faced with aluminum. It is fire resistant and reduces duct noises.
These are just a few of the potential noise issues that you may face in dealing with your furnace. Attempting to fix simple mechanical problems may be to your advantage in saving costs, but if you have any serious concerns about the origin of the noise, consult a qualified HVAC professional to be safe.