Loud Noises in an HVAC Duct

If your heating, ventilation and air conditioning, or HVAC, ducts or ducting network makes loud, rattling noises, try troubleshooting to determine the cause. Your ductwork may vibrate, hum or thump. At times, you may hear pops and bangs or high-pitched sounds coming from your ductwork. Tackle the problem by carefully inspecting your HVAC ductwork to locate the source of the noise.

When trying to assess what's making the noises in your HVAC ducting system, determine its origin. This may not be easy, as echoes throughout the ducting system can distort sounds and make them appear to come from different locations. Start with the compressor or blower area. Your fan may be defective or bent and hitting a part of the fan housing, sending a jarring metallic sound throughout the system.

If you notice loud, sudden pops or bangs when your HVAC system first kicks on, you may be hearing the sound of pressure changes in your ductwork. This type of noise is especially common in metal ductwork systems. Negative air pressure results in the ductwork sucking inward, causing the ductwork to make a popping sound. The process repeats itself when the system is shut off. Relief of pressure in the ductwork, especially in the return vents, makes the system pop when it returns to its normal state.

Vibrating or thumping noises in your HVAC ducts may be caused by a loose nut or bolt in the blower wheel, according to Hannabery HVAC. Carefully inspect the blower wheel and its housing components to determine whether a bolt has sheared off or worked loose. After turning off the motor and the electrical source to the unit, inspect the blades and turn them by hand to see if they're hitting something while the fan turns. In some cases, such pressure changes may be caused by clogged filters, weakened ductwork and ducts that are too small for the HVAC system in use.

Humming noises in your HVAC ducting system are usually caused by the transformer. Most HVAC systems have some humming when in use, but excessively loud humming should be looked at, just for your peace of mind. The transformer is a relatively small unit that transports electricity from the home's power source to the HVAC system.

Growling or rumbling sounds coming from your HVAC ductwork are generally limited to gas or oil-combustion HVAC units. The rumbling sound often originates in the appliance burner and should be dealt with by a professional HVAC technician.