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How to Troubleshoot Strange Noises Coming From Your HVAC System

Modern residential HVAC systems are impressively silent. You may notice a whoosh or a click when a system cycles on, but that should be about it. Louder and persistent noises are often the telltale signs of an underlying malfunction. Even minor issues can lead to higher energy bills and bigger repair costs down the line. Let’s explore some of the most common HVAC noises, what may be causing them, and what you should do about them.

Whistling

An airflow-related whistling sound is usually due to the HVAC filter or a supply register. It may mean that the air is moving around the filter rather than through it. This can happen because you’re using a clogged filter. The average household has to replace its HVAC filter every one to three months. Otherwise, it will eventually become saturated.

Whistling can occur when the filter dimensions are too small for your return register. Another potential issue is the minimum efficiency reporting value (MERV). The industry uses this rating to evaluate HVAC filters. The higher the MERV rating, the better the filtration, but also the higher the airflow restriction. If the MERV rating is higher than what your system supports, it’s effectively the same as using a clogged filter.

If the noise is coming from a supply vent, it generally indicates an airflow restriction. Ensure that the vent is fully open with no obstructions outside or inside the vent. Unless you have a zone control system, all the vents in your home should be open and clear. Beyond that, the noise may indicate that static air pressure is too high. You’ll need to call a technician to perform an inspection. It could be a blower issue or an obstruction in your ducts.

Outdoor Banging or Thumping

Households with a ducted or ductless air conditioner or heat pump will have outdoor equipment as well as indoor hardware. The outdoor unit houses the compressor, condenser coil, fan, and other components. Often, this is the loudest part of an AC or heat pump, but you shouldn’t notice it much from inside. If you do notice it, it may indicate a problem. Loud banging or thumping, in particular, may mean that the compressor is hard starting. This is typically due to a compressor malfunction. Continued use can cause serious, even irreparable damage and potentially, recurring tripped circuit breakers.

Indoor Banging or Thumping

An HVAC system can create a bang when it cycles on due to ductwork expansion and contraction. While this is not usually a sign of a serious problem, you should have a technician look at it. Fuel-fired furnaces and boilers can cause banging sounds at start-up due to delayed ignition. Gas builds up in the chamber before ignition and then combusts suddenly, creating an explosion-like sound. This is an emergency. Stop using the system, and schedule a heating repair immediately. There’s the potential for further damage, fire, and carbon monoxide exposure.

Rattling

Rattling is among the most common and earliest signs of loose hardware. You can generally continue using the system, but you need to schedule diagnostics as soon as possible. In some cases, rattling is due to debris in either the outdoor fan or the indoor blower. It can also be due to loose screws and other connections that cause structural vibration. If you have loose duct panels or mounts, the rattling can be quite loud and persist while the air is blowing. With other loose equipment, you may notice that the rattle comes and goes, even cyclically.

Screeching or Squealing

Squealing or screeching sounds are typically related to motors or other moving parts. They can originate outside, such as from an AC compressor fan. Yet such sounds are most common indoors due to the blower motor or fan. The problem can be as simple as a worn ball bearing you need to replace. If you have an older belt-driven system, it may indicate that it’s frayed or otherwise worn and needs replacement. A lack of lubrication and other motor issues can be quite serious due to the subsequent damage they can cause. You should turn the blower off until you can have a technician examine it.

Hissing

Hissing sounds can be due to airflow obstructions. They may also indicate leaky ducts that can undermine comfort, air quality, and energy efficiency. The most serious cause of hissing is a refrigerant leak in an air conditioner or heat pump system. This is not solely a cooling repair issue. Heat pumps use refrigerant to heat homes in winter as well. Note that you won’t necessarily see refrigerant leak because it’s an invisible gas. If there’s a potential for such a leak, stop using the system right away and call a technician for help.

Buzzing or Humming

Noticeable buzzing or humming generally indicates an electrical component failure. It can be as simple as a worn electrical wire or loose connection. A failing capacitor may buzz or hum, and you may hear such noises due to electrical motor malfunctions. While such problems are generally minor, there is the potential for fire and other electrical hazards. For that reason, you should turn your HVAC system off and wait for the HVAC technician to arrive.

Repeated Clicking

Furnaces often click when cycling on due to their electronic ignition systems. This is not exclusive to electric furnaces, as most modern fuel-fired furnaces have them as well. An isolated clicking sound is usually not cause for concern. Repeated clicking is, as it indicates either ignition or electrical trouble. If it’s not the ignition, it may be a bad relay or faulty control board.

Gurgling or Bubbling

Bubbling and gurgling are typically due to drainage issues. Air conditioners and heat pumps require drainage due to condensation in cooling mode. High-efficiency furnaces and boilers require condensate drainage as well. You need to schedule drain line cleaning but can usually continue to use the system. If the drip pan fills up, the wet switch will trip the system. Check the drip pan regularly to be safe while waiting for the technician.

HVAC Diagnostics, Maintenance, and Repairs in St. Charles County

Based in O’Fallon, MO, Hoff Heating & AC is a fully licensed and insured HVAC company with over 40 years of experience serving homeowners. We provide seasonal heating and cooling tune-ups that can help you avoid unusual noises, high energy bills, and many other problems. Our NATE-certified technicians service all HVAC brands and models. Our team performs warranty and non-covered repairs and is available after hours for emergencies.

To schedule an appointment, call our team at Hoff Heating & AC now, or book an appointment online.