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Furnace Making Intermittent Noise? 4 Common Causes (+ Fixes)

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A furnace is prone to frayed blower belts or general malfunctioning over time. It can produce a wide range of noises. For your average unit, furnaces are crafted to be durable and last up to 20 years with maintenance; however, as with any technology, furnaces can start to make noises that can be worrisome.

Common causes of intermittent furnace noises are a broken blower wheel, unbalanced blower wheels, and dirty burners. Other issues include damaged blower motors, air leaks, or a cracked heat exchanger. Cleaning debris or retightening parts can help, but major issues will require an HVAC professional.

I’ll point out the most common issues that can start producing an audible noise in your HVAC system. Aside from that, I’ll also give you a breakdown of how you can repair any issues that occur. So, let’s discuss further why a furnace may be producing intermittent noise.

Heat Exchanger

A heat exchanger can produce an audible intermittent clicking noise if a crack has formed. Cracks are common for older models due to years of temperatures rising and dropping again. As with most metal materials, this can cause dents, cracks and alter the exterior’s appearance. 

Aside from appearance, heat exchangers that are damaged will leak carbon monoxide. A dangerous chemical such as carbon monoxide is deadly to you and your family’s health. If carbon monoxide is detected, a professional HVAC professional will need to repair it, or the unit will need to be entirely replaced.

How To Fix

  1. Referring to your unit’s instruction manual can help you locate its location.
  2. Check the unit by removing your furnace’s heat exchanger from the unit. 
  3. Wipe down any rust by using a cloth so that you can properly inspect the cracks.
  4. Attempt to identify carbon monoxide leaks by looking for discoloration around the cracks, as this signifies the presence chemical. If you see this, you should immediately stop and call a professional.
  5. If not, continue cutting a piece of sheet metal large enough to cover the cracks with an inch or two of additional space around each edge.
  6. Using a welder, molt the sheet metal to the heat exchanger before allowing it to dry.

Given the complexity involved with this fix, I highly recommend contacting an HVAC professional or having the whole HVAC system replaced. While homeowners can do this job with extensive maintenance experience, it may not be recommended for those lacking in electrical experience.

Motor Bearings

An intermittent sound that produces a high pitch squeal is usually related to ball bearings on your motor. Most bearings have been expertly crafted to last the lifespan of your unit, but a mixture of overheating and unbalancing can cause early damage.

Debris and blockage can cause your motor to overheat. After repeated incidents, this can cause your motor bearings to die due to capacitors becoming clogged and damaged. Luckily, these fixes aren’t too severe and can be fixed with limited HVAC knowledge.

How To Fix

  1. Turn the motor bearing off by using the interlock or thermostat setting. 
  2. Identify broken or misaligned ball bearings by manually moving the blower fan and listening for audible noises.
  3. Check to see if the blower rotates in a full circular motion. 
  4. If it doesn’t and overturns more than a few times, then it’s unbalanced.
  5. Balance the blades by cleaning off the fan blades using a toothbrush, damp cloth, or metal wire brush.
  6. After you’ve balanced the bearings, test the unit by spinning the fan blades again. 
  7. If they don’t spin, then you’ll need to get the unit checked out by an HVAC professional or replace the model unit.

Burners

Louder noises such as banging noises can indicate a dirty burner. Dirty burners cause a delay with the ignition. Gas tends to build up when this happens instead of auto-releasing. Debris, dirt, or even a build-up of soot can be responsible for clogs in burners.

This phenomenon is hazardous as it can cause a spark resulting in a small explosion. As you can imagine, this permanently damages the unit and can be dangerous for any surrounding objects. While you won’t be able to repair the unit if this does happen, you can stick to yearly maintenance to help prevent the burners from becoming clogged in the first place.

How To Fix

  1. Power down the HVAC system by shutting off the electrical system (switch) and fuel (located near the oil tank). 
  2. Next, clean out the combustion chamber using a small wire brush. Be sure to remove any carbon buildup, water vapor, and soot.
  3. After that, inspect the flue pipe for holes or cracks. 
  4. If holes are found, cover them with foil tape.
  5. Replacing the oil filter can stop any clogs in the oil nozzle, so be sure to replace old or dirty filters. You can refer to your user’s manual for filter dimensions, model, and the replacement filter size.
  6. Lastly, test the burners to ensure that they are efficiently working. This can be done by attaching a combustion analyzer, which can help measure the exhaust flue gas. 
  7. If any of these tests fail, then you’ll need to contact an HVAC professional to help repair the unit.

Here is a video demonstration by that shows you what you’ll need to do during your yearly HVAC system maintenance:

Transformers

The last problem that can cause intermittent noise is a low humming sound. This is generally an indication of the furnace’s system starting to break down. Transformers are designed to last for the lifespan of the unit. However, due to improper care, they can begin to break down.

As stated before, maintenance plays a significant role in the longevity of your furnace. Transformers will need to be adequately checked by an HVAC professional when it comes to damages. Although, you can perform simple replacements yourself without hiring a professional.

How To Fix

  1. Start by powering off your furnace by cutting off the electrical current.
  2. Go ahead and open the furnace cover and locate the transformer.
  3. Take a pair of wire cutters and cut each wire that is located on the transformer.
  4. A screwdriver should also be used to unscrew the old transformer before installing the new one.
  5. Transformer models will differ by furnace company, so understanding which one you need is necessary for replacements. Using your user’s manual, you can identify which model is installed.
  6. Next, take the new transformer and attach it to the furnace by connecting the wires to the supplementary wires.
  7. If you’re still having trouble, you can reference the old wires to help you get the correct alignment.
  8. Lastly, close the unit back up and test to see if the transformer is working. 
  9. If not, you’ll need to contact a professional to correctly re-install or replace the HVAC unit.

Here’s a quick video that teaches you how to test a transformer for accuracy:

Final Thoughts

It can be hard to determine if that intermittent sound is an indication of a broken part or a failing unit. However, taking the necessary steps can give you a better idea of how you can keep your unit operating. Yearly or bi-yearly maintenance can help prevent most of these issues from occurring.

Some issues can be dangerous for you and your home. It’s not worth the risk to attempt to figure it out yourself, so contacting an HVAC professional is recommended. To find a trusted HVAC service near you, consider filling out the form listed below.