Furnaces can rattle for a variety of reasons, from clogged air filters to dirty test lights to faulty motors. Those wondering, “Why is my furnace so loud?” will want to take the time to address this question.
Q: My furnace is making a rumbling noise and I don’t know what to do. Why is my furnace suddenly rumbling so much? Is this something I can fix myself, or do I need to hire someone?
A: When trying to determine “Why is my furnace so loud when it’s running?” Many homeowners may find it as simple as replacing a clogged air filter. But the solution isn’t always that simple. A loud furnace may be due to something more serious, such as a faulty motor. Homeowners can assess some of these causes and correct them themselves, but call a professional HVAC technician to make sure the problem is properly diagnosed and addressed in a timely manner.
A loud knocking noise from the furnace may indicate a dirty burner.
Furnaces use a burner that combines air and fuel to create a flame that heats the air. This flame requires an igniter to turn the air/fuel mixture into a flame. The igniter can be a pilot light or an electric igniter that produces a spark to burn the mixture.
When the burner becomes dirty, the igniter may take longer to burn the air/fuel mixture. If this happens, the air/fuel mixture will build up to the point that when it does ignite, it will make a loud crashing sound. Unless the homeowner has experience with gas burners, the job of cleaning them should be left to a professional. Otherwise, the home is at risk of being left with a gas leak that could be quite serious.
A loud rumbling sound from a gas furnace means that the ratio of air to fuel may be unbalanced.
In order to function properly, the furnace must have the correct air-to-fuel ratio so that the burners ignite without delay and heat the air as specified. When the air-to-fuel ratio is unbalanced, it can cause delayed ignition and a loud roaring sound when fuel builds up. Adjusting the air to fuel ratio is not something a homeowner without training and experience should attempt. It can lead to excess gas in the home, which can lead to more problems. It is important to get a licensed HVAC technician to evaluate and adjust the air to fuel ratio so that the furnace operates properly without any rumbling sounds.
A dirty pilot light can cause the furnace to make a loud noise.
Just like a dirty burner, a dirty pilot light can interfere with the ignition of the burner. If ignition is delayed, fuel will build up; when it finally ignites, it will cause a loud noise. When dirt builds up on the pilot, the pilot may have trouble staying lit or may be too weak to ignite the fuel. Cleaning a dirty pilot can be as simple as blowing air through it with a can of compressed air. However, if this does not work, it is necessary to leave the task to a technician who can take the pilot assembly apart to clean it.
A clogged air filter can cause a whistling sound.
The air filters on your furnace clean the air that is drawn in, heated and pumped into the room by the furnace. When these filters become clogged, the furnace becomes less efficient, resulting in less airflow. This can cause the furnace to make a loud buzzing or whistling sound. Homeowners will want to change their furnace air filters regularly (every 1-3 months for 1-2 inch filters and every 6-9 months for 3-4 inch filters) to ensure the system is working at maximum efficiency. If changing the filter does not stop the furnace from making a loud humming noise, it is recommended that the furnace be inspected by a licensed HVAC professional.
Holes or seams in air ducts can cause unexpected noise.
Metal ducts installed to direct air from a furnace or air conditioner to an air outlet are designed to contract in cold air and expand in warm air. As a result, if the heating equipment has been out of service for some time, it may make unexpected noises as it passes through the ducts. However, if these noises occur frequently or become louder, the ducts may need to be replaced. There may also be a loose joint or screw in the duct, in which case there may be an unexpected noise as air passes through the duct.
Motor failure can cause the furnace to make noise.
All furnaces are equipped with a blower motor, which sends warm air from the furnace into the ducts and through the vents to the rest of the house. If the belt on the blower motor becomes weak, it will make a noisy “hissing” or “whirring” sound. Replacing the belt will fix the problem, but it’s not a task you can do yourself. Likewise, if it sounds like metal scraping metal, the motor may have a more serious problem (e.g., a loose or damaged blower wheel). If you hear such a sound, it is recommended that you turn off the furnace immediately and have a professional technician inspect the furnace.
The problem may be a loose vent cover.
The vent cover may be located on the floor, wall or ceiling. If the vent cover is loose, you may hear a rattling sound as air passes through. There may also be a loud noise when the unit is turned off, which may be caused by the cover returning to its original position when the air stops flowing through the loose vent cover.
Use a screwdriver to tighten the screws securing the vent cover. At this point, be careful not to over-tighten the screws, but to tighten them so that the vent cover does not rattle. There are many vent covers in the house, so check them one by one and find the one that is making the rattle.
A rattling furnace fan may be caused by a broken heat exchanger.
The heat exchanger is the part of the furnace that heats the air. When heat builds up due to poor air flow, for example due to a clogged air filter, a dirty burner or a dirty pilot light, cracks can develop in the heat exchanger. When cracks appear in the heat exchanger, the hot air expands each time it hits the crack, which can cause a loud noise. Although cracks in heat exchangers are rare, any suspicious cracks should be addressed as soon as possible because harmful gases such as carbon monoxide may be released into the home. This inspection should be performed by a professional who can accurately diagnose the cause of a loud furnace fan. A cracked heat exchanger usually means it’s time to replace the furnace.
An HVAC technician can fix the problem of a loud furnace.
Some problems with a noisy furnace can be remedied at home, such as replacing a clogged air filter or tightening a loose vent cover. However, other problems, such as cleaning a dirty pilot light or adjusting the air-fuel ratio, can be more difficult. Homeowners who are concerned about diagnosing a noisy furnace, such as a new furnace that is louder than an old one, or who want to save money on a long-used furnace, are wise to seek the help of a licensed HVAC technician. These professionals are trained to work with a variety of furnace systems, and they know how to assess issues such as a furnace making loud noises. In the long run, this experience may save you time and furnace costs.
