Woman being kept awake by furnace noises at night

4 Furnace Noises (And What They Might Mean)

Background noise is a reality of modern living. From the buzz of your refrigerator’s compressor to the low hiss of your water heater, we hear a lot of sounds around the home. We become so familiar with these household noises that for many of us, a new or different noise from an appliance is our first sign that something’s going wrong. This is especially true when it comes to your heating and air conditioning systems! In this article, we’ll look at four common furnace noises and what they might mean for the health of your heating system. Remember that the only way to be sure of what issue your furnace has is to book an expert inspection. These tips should help point you in the right direction and assist you when describing the issue to your HVAC professional and deciding if furnace repair is needed.

Furnace Makes a Whistling Sound

One of the most common unpleasant furnace noises is a whistling sound. Whistling furnaces can be shrill and may be quite loud or fairly subtle depending on the type of furnace and the nature of the issue itself.

First, try to identify roughly where the sound is coming from. If you hear whistling in one room but not in any others, it may actually be a blockage or air leak in your air ducts or a partially closed air vent rather than an issue with the furnace itself. Sometimes, if a new furnace fan or furnace has been installed recently, the volume and velocity of air being blown through your home’s air ducts will increase. Normally this isn’t an issue, but in some cases the increased air pressure can expose gaps in your home’s ducts and create a whistling sound.

The blower itself could have a mechanical issue causing squeaking or whistling sounds, as well.

If the whistling sound comes from the furnace and not from a vent or duct, the problem is usually an airflow issue, though it could also be a mechanical problem with the blower itself. As a furnace blows warm air throughout your home, the air passes through a filter, then into your ducts and out of your vents. A clogged or dirty filter may reduce airflow through your system and cause whistling noises as the air forces its way through the filter. Loose bolts or other fittings on the furnace itself may also cause rattling or whistling noises.

One final possibility is that your furnace has a gas flow issue. A damaged or defective gas valve could be the culprit. Always leave gasfitting and valve repairs to a trained professional! Issues related to gas flow can be dangerous and should be inspected and fixed as quickly as possible.

Furnace Makes a Loud Buzzing or Humming Noise

While all furnaces generate some ambient noise, most of the time furnaces run quiet enough that it is not disruptive. Especially loud buzzing or humming, either when the furnace starts up or throughout its operation, can indicate a variety of mechanical or electrical problems. The main culprit would be an issue with your blower fan motor. If the sound is loudest during startup, this is very likely the reason. The fan motor itself may be dying, or the electrical components powering the fan may be worn out or faulty.

If the issue worn out or faulty electrical components are causing the issue, the buzzing or humming noise will be loudest at startup and typically diminish or stop once the fan is up to speed. This is because the fan draws the most power and is under the most strain when it starts. Once it’s running, the power draw drops significantly, so the noise will typically decrease as well. If it’s a blower fan issue, the noisiness may or may not improve after startup.

Replacing electrical components like a capacitor or transformer is fairly affordable. A service technician can quickly diagnose and replace the faulty component, often on the first visit. Replacing a blower fan is a more involved job that requires removing the existing fan and rewiring the replacement.

Furnace Makes a Banging or Booming Noise

Sometimes it’s hard to tell if a furnace is running louder than it ought to. If your furnace is making banging or booming noises, you’ll probably figure out pretty quickly that something’s wrong!

One cause for banging and booming furnace noises concerns your home’s air ducts. Just like whistling ducts, air pressure problems may be the cause of banging noises in your home. If your air ducts are undersized for the furnace or you have a clogged filter, the air pressure may be too extreme and cause the metal ducts to expand and contract, causing occasional loud bangs as the metal pops in and out of place. This is an annoying problem and may reduce the longevity of your system, but it’s not particularly dangerous to you or your family.

The second cause for banging or booming noises is more dangerous. If the source of the noise is the furnace itself and the noise occurs when the system starts (or shortly after), it may be what’s called a delayed gas ignition. Delayed gas ignitions occur when the gas to air ratio is poor (either too much gas or too much air in the mix) or the ignition is otherwise delayed by a faulty, worn out, or dirty component. Delayed gas ignitions can be dangerous for you and your furnace. Essentially, each banging noise from your furnace is the result of an explosion. This is because the gas cannot ignite until the gas to air ratio is within a certain range. A delayed ignition means that gas has had time to build up. Once it finally ignites, the excess gas burns very quickly, causing a small explosion. It’s like when you light a grill after the propane has been running for a few seconds.

Delayed gas ignitions risk damaging your heat exchanger, which can be very costly to repair or replace. Anything to do with gas flow issues is a safety risk and must be dealt with immediately! Fortunately, you can avoid encountering these issues (and save yourself a big repair bill) by scheduling annual furnace maintenance appointments. Our annual maintenance plan is an affordable way to extend the lifespan of your HVAC systems and prevent potentially dangerous issues like delayed gas ignitions from occurring.

Furnace Makes a Gurgling, Dripping, or Splashing Noise

This one might seem counter-intuitive. How can a fiery gas furnace be making watery noises? In most cases, these sorts of noises come from newer, high efficiency furnaces. Because of the way high efficiency furnaces vent exhaust fumes, they may produce condensation from moisture in the air that must be drained. When there’s an issue in draining this condensation, it may cause gurgling, dripping, or other watery sounds. Usually, these noises are easy enough to sort out. A full or clogged drain pan or a blockage in the drain pipe itself may be the culprit.

If you hear water sounds coming from vents or air ducts instead of the furnace itself, this could indicate a more serious moisture problem in your home. Be sure to get the issue inspected immediately, as moisture can cause very serious issues like widespread mold growth or even structural damage to your home.


Noises can tell you a lot about what’s going on with your furnace. (Furnace smells can give you a clue too!) Use the tips above to get a sense of what the issue might be. If you’re worried there is a serious issue or just want peace of mind that everything’s OK, schedule a furnace inspection or maintenance appointment with Swan!


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