Furnace noise. What does it mean? Some furnace noises are normal – they do not indicate a problem. Others are a sign that furnace repair might be necessary.
Here are common furnace noises, what they mean and what you can do about them.
Humming
There are several reasons for a furnace to hum.
Normal reasons: The draft inducer motor hums when the thermostat first calls for heat. It might sound like a whining noise, but it’s nothing to be concerned about.
The furnace blower motor hums when circulating air. The hum of both motors should be fairly low and consistent without any buzzing sound.
Problems & Solutions: Electrical issues cause humming, whining or light buzzing. They include either motor being worn out and starting to fail, a failing blower motor capacitor or a bad transformer. Also, the loose wiring or fan blade can create a humming sound.
Most homeowners call a service technician to diagnose and solve the issue. If you’re comfortable using a multimeter to check these electrical components to find which one is failing, you might be able to make a DIY repair.
Repair Cost: $25 on the low end for a capacitor or transformer replaced DIY to $250 to $1,050 for pro replacement of a motor.
Buzzing
Your furnace shouldn’t buzz continuously. If the buzzing is new, it indicates a problem.
Normal reasons: The capacitor might buzz for a split second when providing an electrical boost to get the blower motor going.
Problems/Solutions/Repair Cost: Buzzing is almost always an electrical problem. See the information above on furnace humming – it applies here too.
Clicking
The furnace making a clicking sound is one of the most common furnace noises – and it usually indicates an ignition issue. The clicking is the igniter attempting to light the furnace burner.
Normal reasons: You might hear one click when the furnace starts heating – that’s OK.
Problems & Solutions: If the furnace is clicking but not lighting, the problem is likely one of the following –
- The ignition control board is bad
- The gas valve needs to be replaced because it isn’t opening
- The burners are dirty, so gas isn’t getting through them in sufficient quantity to light
- (Older furnaces only) The pilot light is out or the thermocouple is bad
You can check the control board with a multimeter to see if it is getting power. If not, it’s bad. DIY replacement is possible. Otherwise, call a furnace technician to clean the furnace and test the components to determine the repair need.
Repair Cost: Furnace cleaning starts at around $100. Gas valves and boards range from $100 to $500 for the part plus an hour for labor – another $85 to $150.
Banging or Popping
Is the banging noise coming from the furnace or ductwork?
Normal reasons: Furnace banging or popping is not a normal sound, especially if it is a recent development. Some banging in the air ducts might be normal, but it should be distractingly loud.
Problems & Solutions: The most common cause of furnace banging is delayed ignition of the gas.
1). Gas builds up, ignites, and “bang,” you hear a small explosion inside the furnace. Reasons include a poor air-to-gas mix, burners that are dirty or out of position or low gas supply. Low gas supply is common in propane furnaces running off a propane tank – so check the tank, and have it filled if it is under 10%.
When the furnace itself is banging or popping, it makes sense to contact a technician to inspect the furnace, diagnose the problem and give you a repair estimate.
2). The second cause is expanding or contracting ductwork during furnace operation. Sheet metal will pop or bang when it heats up and/or cools down or when air is pushed through it.
Some light banging or clunking in the ductwork is normal. These banging noises can be muffled by insulating the ductwork.
However, if this is a new issue or it is really loud, then you might have an airflow problem – Dirty air filter, dirty ductwork or vents that are closed and should be open.
Change the air filter. Open all vents/grates. If the problem persists, call a technician for an inspection.
Repair Cost: A new filter costs less than $10. Adjusting the air/gas mix costs the price of a service call, usually $85 to $150. Insulating exposed ductwork costs $2.00 to $10.00 per linear foot depending on whether you DIY or not and what insulation type you use.
Rattling, Rumbling, Clunking
Normal reasons: A furnace in good condition should not rattle or make other unusual noises.
Problems & Solutions: There are two main causes of furnace rattling, clunking or whining.
1). Cracked heat exchanger: This is the worst-case scenario. A leaking heat exchanger can release deadly carbon monoxide into your home.
If the rattling or rumbling is most pronounced at the start of a heating cycle, it’s a good bet that this is the issue. The rattling might stop as the heat exchanger warms up, expands and seals the crack.
The fix for a cracked heat exchanger is a new furnace because the repair bill would likely be 50% to 75% the cost of furnace replacement.
2). A loose or damaged part: A bad blower motor bearing, loose panels or connections to ductwork or a loose screw or bolt somewhere will cause rattling, clunking or a vibration noise.
Bad bearings can also cause a squealing, grinding or screeching sound – when metal is grinding on metal. It can produce a high pitched noise that makes you want to cover your ears.
Take the cover off the furnace while it is running, and try to identify the exact location of the rattle. You might be able to tighten a loose part or add a few sheet metal screws to ductwork. If it’s the blower, you can DIY replace it or call a furnace technician.
Repair Cost: Minor issues can be fixed for the cost of a service call. Replacing a blower motor runs $200 to more than $1,000 based on the motor and whether you DIY or not.
The cost of a new furnace installed ranges from $2,750 to $10,000+ based on size, efficiency and performance.
Whistling
This furnace sound is about restricted airflow.
Normal reasons: Your furnace shouldn’t whistle.
Problems & Solutions: There are a few potential problems, each with its own solution.
1). Dirty air filter – This furnace whistling comes right from the furnace itself. Replace your air filter, start the furnace, and see if the problem is resolved.
2). Closed vents/grates. They might whistle if closed but are still allowing a little air through them. This doesn’t represent a problem. If you don’t want heat in a particular room, you might have to close the vent and put a towel over it to muffle the whistling.
3). Damaged ductwork. Locate the exact spot where the ductwork is whistling. If it’s loose, it might need a few sheet metal screws. Wrapping ducts in insulation will also stop the whistling and make your home more energy efficient.
Repair Cost: $10 or less for a new air filter up to $200 to have a technician tighten loose ductwork. Wrapping ducts in insulation runs $2 to $10 per linear foot.
Knocking
This is a catch-all category with several potential causes. Most are not normal and should be investigated.
Normal reasons: The furnace cabinet or ductwork might make light knocking or popping noises when warming up at the start of a heating cycle or cooling down after one.
Problems & Solutions: Here are common issues and what to do about furnace knocking.
1). A loose or broken part. The inducer motor, blower motor or blower fan can loosen over time and knock against other parts or make a rattling sound or vibration noise. In older furnaces with belt-driven motors, a damaged or worn belt will cause knocking or screeching too.
Feel free to inspect the furnace interior for a loose part to see if you can tighten up the bolts. If not, call a furnace technician for the work.
If a part is visibly broken, shut down the furnace until it can be repaired.
2). Dirt and debris. Excessive dirt buildup can cause the blower to work too hard, and it might knock or rattle, especially if the exertion causes it to become unbalanced. Have the furnace professionally cleaned and the blower re-balanced.
3). Dirty air filter: A clogged air filter will knock against the frame that holds it when the blower tries to suck air through it. Change your filter to see if that solves the furnace knocking.
4). Ductwork knocking: Sometimes expanding ductwork will knock into the wall around it. While annoying, it isn’t a problem.
However, if the knocking is caused by loose ductwork, it should be tightened, wrapped with mastic and tape and/or insulated to prevent the knocking from occurring.
Repair Cost: If changing the air filter doesn’t solve the issue, then it is difficult to estimate what the problem is and what it will cost.
Furnace cleaning costs $85 to $200 based on time and prices in your area.
Repairing ductwork costs the same amount per hour plus $5 – $10 per linear foot when a contractor tapes and insulates your exposed ducts.
Squealing , Squeaking, Grinding and Scraping
This high-pitched noise often comes from issues related to the blower assembly in your furnace. While it can sometimes be resolved with simple maintenance, persistent squealing or squeaking may signal a more serious mechanical problem.
Problems & Solutions:
1). Friction of motor bearings: Bearings in the blower motor can wear over time, creating grinding or squeaking noises if not lubricated or if they become damaged.
2). Blower shaft lack of lubrication: Some furnace blower assemblies require occasional oiling. Insufficient lubrication can cause metallic squeaks and squeals.
3). Worn belt of belt-driven blower motors(Older Furnaces): In older systems with belt-driven fans, worn or loose belts can slip and produce a shrill squeal.
To solve these problems, you can :
Check or replace the belt. If your furnace uses a belt-driven blower, look for signs of cracking, stretching, or loosening. Replace the belt as needed.Or you can adjust the tension of the belt. A belt that’s too loose or too tight can also cause squealing.
Lubricate motor bearings. Some blower motors have designated oil ports that allow you to add a small amount of lightweight motor oil, such as 10W non-detergent, to reduce friction; however, it is important to avoid over-lubrication since excess oil can attract dust and cause additional wear or damage. In many modern blower motors, the bearings are sealed and do not have accessible oil ports, which means they cannot be manually lubricated; if you suspect bearing-related noise in a sealed motor, you should contact a professional to determine whether the motor needs repair or replacement.
Replacement Belt Cost: $5 – $30, depending on the size and type of belt.Lubrication: Typically low-cost if you do it yourself; professional service fees may run $50 – $100.Motor/Bearing Repair or Replacement Cost: $150 – $500+, depending on parts, labor, and furnace model.
Grinding or Scraping
Grinding or Scraping in a furnace often indicates serious mechanical issues and typically arises from one of three main causes. In some cases, the blower wheel becomes bent and scrapes against the blower housing. Similarly, motor bearing failure can generate grinding noises due to severe wear on the bearings. Another potential cause is a foreign object, such as debris or a loose screw, getting trapped in the blower assembly.
If you notice a grinding sound, it is crucial to shut off the furnace immediately to prevent further damage. Next, inspect the blower assembly for any foreign objects or bent fan blades if it is safely accessible. Because grinding typically signals significant mechanical problems, calling a professional is often the safest and most effective route for proper diagnosis and repair or part replacement.
Rumbling
The most common issues are gas valve problems.
See “Rattling, Rumbling or Clunking” above for details on furnace rumbling.
See “Rattling, Rumbling or Clunking” above for more details.
Gurgling
Water gurgles, so a furnace gurgling noise indicates an issue with condensation. If you hear furnace gurgling, you almost certainly have a condensing furnace. These are high-efficiency furnaces that transform so much heat from the exhaust that the water vapors in the exhaust are cool enough
Normal reasons: Condensation moving through the drain, drain pump and system are a sign the furnace is working as it should.
Problems & Solutions: If the furnace is shutting down or if you see a water leak around the pump and line, then it might not be getting rid of the condensation. The pump could be bad, or the condensation line could be blocked.
Repair Cost: Cleaning out the drain line costs around $125 to $215. A new condensate pump runs $160 – $300 for the part plus about $125-$165 to have installed.
Thumping or Bumping
Thumping or bumping noises in a furnace often indicate something is out of alignment or loose. The most common causes include an unbalanced blower wheel, which may create a rhythmic thump if out of balance, or a loose motor mount or blower assembly, allowing the unit to bump against the housing.
To troubleshoot, inspect the blower components for any debris or bent blades, and secure motor mounts by tightening loose screws or brackets. If the thumping continues, a professional can balance or replace a bent blower wheel to restore proper alignment and eliminate the noise.
Beeping
Newer furnaces and thermostats beep to notify you that:
The air filter is due for a change
The thermostat battery is low
The system is due for regular maintenance
A sensor is dirty or is not working
Check the obvious issues first – filter and thermostat battery. If those don’t resolve the problem, it’s probably time to have the furnace checked, cleaned and tuned for optimal performance.
