Thumping Noises in HVAC Units

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Determine the source of thumping in your HVAC system.

When the weather is hot or cold outside, you turn on your air conditioner or heater for relief. The last thing you want to hear is alarming noises coming from your air conditioning or heating system. Troubleshooting thumping noises in your ventilation system isn't always easy. Take a direct approach to determining the source, the cause and the remedy.



HVAC is a term used by ventilation technicians that defines one system used for heating, ventilation and air conditioning. It's a term used to define both air conditioning and heating units installed in homes and buildings. HVAC systems are favored because they allow you to use the same unit for your air conditioning, heating and ventilation needs through a condenser, a compressor and a condenser coil. An air handler or a furnace is also fitted with an evaporator coil indoors for heating systems, and a compressor is used to pump cooling refrigerants into the system for cooling. The heating and cooling systems share ducts that provide airflow to every area of the house.


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Thumping noises in your HVAC unit may be caused by several different things. Determine the source of the thumping noise, which may be challenging depending on the size of your HVAC system and the distortion of sound and echo in large basements or buildings. Thumping may be caused by something out of balance at the unit's source, such as the fan or the fan blower wheel. The motor mountings may also be loose, allowing the motor to vibrate and thump against the motor housing. Thumping noises may also be caused by a bent fan blade hitting some other part of the housing.



Coming from your ductwork, thumping noises may simply be caused by air pressure changes within the ducts when hot air is blown into cool ductwork or cool air blows into hot ductwork. Inspect your ductwork for loose fittings, broken seals or holes in your ductwork that may be the cause of the thumping noises. Loose ductwork may bang against something as air flows through them, so inspect the ducts when possible with the air or heating on to offer easier identification of the source of the noise.



Before you attempt to fix any thumping noises in your HVAC unit, cut the power to the unit at its source and in your circuit breaker panel. Inspect screws used for mounting the motor and the blower fan and the fan itself. Tighten loose screws or identify bent or otherwise damaged components. If you're uncertain how to repair the damage, contact a trained HVAC technician.



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