Duct noise is rarely dangerous, but it is often a symptom. Many of these sounds come from too much air being forced through too little duct, which also means the system is working harder than it should. Here is what each sound usually means.
Popping, banging or booming
Usually heard when the blower starts or stops. Sheet metal ducts flex as air pressure changes, and a panel pops in or out, a bit like an oil can. It often means:
- Undersized or restricted ducts, so pressure spikes when the blower starts
- A closed or blocked return, or a clogged filter pulling the return duct inward
- Too many closed supply registers
- A blower set to a high speed for the duct system
Thermal expansion also causes ticking or popping as metal ducts warm and cool, especially in hot attics. A little is normal. Loud booms at every startup are not.
Whistling
Air squeezing through a gap or an opening too small for it. Common sources:
- A clogged filter
- An undersized return grille, very common in older Sacramento homes
- Partly closed registers or dampers
- A leak at a duct joint or around a register boot
Start with the filter and registers. If the return still whistles, it is likely undersized. See what size ductwork you need.
Rattling or vibrating
- A loose register or grille. Tighten the screws.
- A loose damper inside the duct
- Duct straps that have come loose in the attic
- Debris inside the duct
- Vibration from the blower carried into the ducts where there is no flexible connector at the unit
Rumbling or roaring
A steady low roar through the ducts usually means high airflow velocity. The system may be oversized for the ducts, or the blower speed set too high. It can also come from the air handler or furnace itself.
Scratching or scurrying
Animals. Rodents in attic ducts are not unusual. They also damage flex duct and chew wiring. Deal with the animal first, then have the ducts inspected for damage and contamination.
Why duct noise matters
Noise from too much pressure means restricted airflow, which cuts capacity, can freeze the coil in summer, overheat a furnace in winter, and wear the blower motor. Measuring static pressure tells us whether that is happening. A new, larger system on the same small ducts usually makes the noise worse. See what size AC you need.
What you can try
- Replace the filter.
- Open all supply registers and clear the return.
- Tighten loose register screws.
- Note when the noise happens: startup, shutdown or continuously.
What we do
We measure static pressure and airflow, inspect the ducts in the attic, and check blower settings and the return size. The fix may be adding a return, adjusting blower speed, sealing leaks or replacing a restrictive section. Our diagnostic is a flat $69 and it is credited toward the repair. See duct sealing versus replacement and our duct work services. If the noise is the equipment rather than the ducts, see AC noises or furnace noises.
Frequently asked questions
Why do my air ducts pop or bang?
Pressure changes flex the sheet metal when the blower starts or stops. It usually points to undersized or restricted ducts, a blocked return, a clogged filter or too many closed registers.
Why is my air vent whistling?
Air is squeezing through a small opening, often from a clogged filter, an undersized return, partly closed registers or a leak at a joint.
Is it normal for ducts to make noise?
A little ticking from metal expanding and cooling is normal. Loud booms, persistent whistling or roaring usually point to an airflow problem.
Can closing vents make ducts noisy?
Yes. Closing too many registers raises pressure in the duct system, which can cause whistling, popping and reduced airflow.
Why do I hear scratching in my ducts?
Most likely rodents or other animals. Have the animal removed, then have the ducts inspected for damage.
Will a new AC fix noisy ducts?
Usually not. A larger system on the same ducts often makes the noise worse. The ducts and return need to be sized for the system.