You furnace is responsible for keeping your home warm on chilly nights. With all that is going on inside a furnace, there is no wonder it makes all sorts of noises. But when you can hear a loud humming sound that doesn't appear to be part of the unit's normal operation, there is a possibility the furnace has a problem. Recognizing the possible causes of the humming noise will allow you to troubleshoot and potentially solve the problem before it interferes with the comfort level of your house.
M/F Sometimes a humming noise coming from your furnace is the result of loose mounting brackets. If the noise is coming from a recently installed furnace, perhaps the installation crew did not secure it properly. The vibration caused by the fan or other parts could simply be forcing the entire unit to move, causing a low humming noise as it rattles against the floor or surrounding support structures. Even if the furnace is not new, loose mounting brackets could still be the cause. Examine all mounting brackets, and tighten nuts and bolts where needed to see if it reduces the noise. The movement of the unit could have worked it loose over time.
M/F A flame sensor is a mechanism inside a gas furnace that indicates whether the pilot light is staying lit. These are standard in some models and are added to others when the pilot has a tendency to blow out often. If this sensor gets dirty, a clicking and humming noise can be the result. Review the literature on your furnace to find out if yours has a flame sensor and where it is located. If you find it is dirty, turn off the system and clean it to restore proper functioning.
M/F When the furnace's blower forces air through the ducts in the home, the force of the air can be enough to vibrate the sheet metal on the ducts themselves, which is especially true when the ducts are loose or flex easily. You can tap on your ductwork to see how easily the metal moves. The metal bending in and out easily is not uncommon. If it does, the air will certainly cause the metal to vibrate, creating a hum. You can dent the metal slightly to make it less flexible or even put rivets in the metal to make it more rigid.
M/F If your furnace has a low humming noise coming from the unit even when it's not in use, there is a good chance it is the sound of your low voltage power. Furnaces generally have a power switch located somewhere on the furnace that will cut the power to the system. If you are bothered by the humming sounds, flipping this switch off during nonuse should stop it.