Gas furnaces can sometimes make some natural creaks and sounds while running. But the sudden appearance of loud sounds quickly after the unit starts can indicate a major problem that needs to be dealt with immediately to make sure the furnace doesn't suffer permanent damage and that there aren't any potential safety issues.
The type of noise is often a good indicator as to what is going wrong. Use this guide as a general reference to what is potentially the problem, but leave the maintenance and replacements up to a professional heating repair technician.
Grinding Noise: Loose Bearings in Draft Inducer
Does the furnace make a loud grinding noise? The bearings on the draft inducer blower might have come loose.
The draft inducer blower is a type of motorized fan located above the furnace's burner and circulates the incoming air that's needed to fuel the combustion system. Bearings are what hold the inducer steadily in place even as it blows the air around. The bearings can gradually loosen over time due to the vibration or due to wear.
It's best to replace loose bearings entirely instead of simply tightening the existing bearings. Microscopic cracks could exist on the older bearings that would cause the bearings to fail at a later date despite looking healthy.
Metal on Metal Noise: Blower Wheel Malfunction
Does the noise sound like metal scraping up against metal? The malfunction is possibly in the blower wheel, which is another important fan that keeps air circulating in order to stabilize the furnace and warm your home. A few different blower wheel problems can cause the sound.
The simplest answer is that the blower wheel is coming loose from its shaft and only needs to be tightened back into place. But the drooping wheel can also be due to a broken mount, which means that the entire assembly is clanging and at risk of falling off into your furnace. The metal on metal sound could also stem from the blower wheel having a broken part or parts that are clanging as the fan makes each pass.
Call in an HVAC technician (such as one from Alliance Heating & Cooling LLC) as soon as you hear the metal on metal sound. Keep the unit off until a tech can diagnose the problem to avoid any major part failures or further damage.
Loud Bang: Dirty Burner Causing Excess Gas
Banging noises aren't uncommon in the operation of older gas furnaces or furnaces attached to a metal, aging duct system. But a sudden uptick in the frequency or volume of the banging can indicate a new problem.
The banging could be essentially tiny explosions happening due to excess gas in the system. The gas can accumulate if the burner is overly dirty, which fails to send the proper signals to the system in order to control gas intake and the flame source.
Turn off the electric and gas to your heater until you can have a service tech take a look. If you still smell gas, call the gas company immediately and vacate your home until the gas company has said the situation is safe.