Furnace noises are often a sign of problems either with the unit itself or with the way air is flowing through your furnace. If your system is making loud noises when it used to be silent, you should have a professional inspect the furnace for any causes of damage. A loud banging noise can indicate a minor problem or be a symptom of something very serious.
Heat exchangers are the metal components in a furnace that channel the heat created into the air. Heat exchangers are designed to withstand very hot temperatures for extended periods of time, but they can eventually crack or break down. When this occurs, the metal parts of the heat exchanger will continue to expand when the furnace is turned on, but the crack will cause a discontinuity, resulting is a loud bang as the exchanger adjusts.
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In an oil furnace, a loud bang may be the sign of excess oil substances that have become trapped in the bottom of the furnace and are igniting when you try to use the furnace again. This is a very serious problem, not only because it creates a fire hazard but because it can easily destroy the inside of your furnace. If the noise is coming directly from an oil furnace, shut down its power and call a professional.
Every furnace has a blower designed to propel air from the furnace into the air ducts of the house. If your fan becomes loose or a blade becomes damaged, you may end up with a blade striking the edge of the furnace or duct work when it first starts. This situation can create the banging noise you are hearing, especially if it echoes throughout the ducts. You will need to replace the fan to resolve this problem.
The noise may not be coming from your furnace at all. In some cases, if ducts become unsealed, misaligned or improperly installed, the changes in heat and air flow can cause them to sway or resettle, resulting in banging noises throughout your system. You can reattach and repair ducts to solve this problem.