How To Fix It

Why Air Vent Is Making Noise Even When It Off?

It is normal for you to hear a humming sound from the air vent when the HVAC system is operating. But if the noise continues even after the equipment gets turned off, then:

  1. Your fan is set to ‘auto’
  2. There is an electrical issue within the system that keeps it running even when turned off
  3. There is a problem with the thermostat
  4. You have leaky ducts
  5. Your HVAC system is infested

Keep on reading to find out what’s producing the noise in your specific case.

Why Air Vent Is Making Noise Even When It Off?

The noise you’re hearing might be coming from the indoor unit’s condensate pump. If the HVAC system has been running on an extra humid day, the element will need a bit more time to drain the excess moisture out of the system.

However, if your air vent continues making noises even after a couple of hours have passed, then one of the following might be to blame:

Fan Set to Auto

The thermostat has two settings to control the HVAC system’s fan – ‘on’ and ‘auto’. The latter means that the fan will turn on automatically only when the unit is cooling or heating the air.

However, if you switch the fan mode to ‘on’, the element will constantly circulate air through the system. This can make the air vents in your house make noise even when the rest of the equipment is off.

Read: How To Fix Insulation Blowing Out Of Air Vents?

Electrical Issues

An electrical problem can make air continuously come out of the vent even when the HVAC unit is turned off. This will not only cause unnecessary noise, but the system will also still be consuming energy.

A problem with the contractor

The contractor is a component in an HVAC system that controls the electricity flow. It, basically, supplies the power and cuts it off whenever it’s needed.
If the contractor goes bad due to a wiring issue or old age or if the element gets stuck, the component might fail to cut the electricity off, so the HVAC unit will continue to run.

A problem with the capacitor

The starting and running capacitors are responsible for powering up the motor and keeping it going. These elements maintain a constant electrical supply, and if they ever go bad, they might end up sending power to the motor even when it’s not needed.

Read: Does Closing Air Vents Help Cool Other Rooms?

Have a Question? Ask HVAC Technician

Click here to use the chatbox to speak with one of our technicians.
No in-home service calls. No appointments.

Wiring issues

If the elements in the HVAC system are not properly connected, the unit might fail to either kick in or turn off.

If you feel comfortable working with electricity, you can attempt to check the wires. Simply make sure that all the connections are secure and that the wires are not damaged.

Do bear in mind, however, that if you suspect an electrical issue, it is always better (and safer) to invite an HVAC expert over to deal with the problem.

Read: Why Dust Coming Out Of Air Vents?

Thermostat Issues

The thermostat is the device that tells your heating and cooling system when to turn on and when to stop operating. If the element goes bad, it might fail to turn the unit off or the system will continue to run even after the set temperature has been reached.

The thermostat has failed

The device can go bad due to general wear and tear (it is recommended to replace your thermostat at least every 10 years) or improper installation. A low-quality device can also fail in the shortest timeframe.

There is an issue with the contractor or relay switch

Thermostats communicate with a contractor or a relay switch which, in their turn, send the commands to the rest of the HVAC equipment. If the thermostat is unable to communicate with this essential element, the component might continue supplying power to the heating and cooling system.

The batteries have died

If you have a battery-operated device, you have to make sure that the batteries are fresh. Otherwise, the thermostat won’t be able to fulfill its functions.

In general, you might need to change the batteries once a year.

There is a wiring issue

If the wires in the thermostat have disconnected (this can happen over time due to vibration) or if they are connected to the wrong terminals, the device won’t be able to operate.

Read: Do You Have To Vent A Portable Air Conditioner In Heat Mode?

Disjointed Ducts

If the ducts are not sealed correctly or are disjointed, the negative air pressure can suck in dirty air. As a result, you’ll have dusty air circulating through the system and your air vents might become noisy as air will be entering the system, even when the equipment is off.

Duct leaks usually develop in crawlspaces, basements, attics, and garages – the spaces in the house that do not get checked as often as other rooms. These are also, typically, the most drafty spaces in the house, so you’ll have air entering the HVAC system practically all the time.

Thankfully, you can seal leaky ductwork on your own using mastic sealant or metal tape.

To find the leaks in the first place, you can inspect the ductwork and the vents for visible holes and loose connections. You can also take an incense stick or a piece of toilet paper and slowly pass it over the ducts. Once the smoke or the paper begins to move, you have found a leak.


If you hear scrabbling or buzzing inside the air vent, then it looks like you have guests. Unfortunately, insects and small animal infestations are quite common in some areas. And, in a lot of cases, they will choose to make your HVAC system their ‘home’.

Open up the air vent and have a look inside, if you see any insects, nests, scratches on the vent walls, or other signs of an infestation, then you would have to immediately call a professional to deal with the problem.

The worst part is that the animals or insects can get into the heating and cooling from any opening (not just the air vent), so to prevent such a problem from occurring in the future, you would have to:

  • Always keep screens and covers installed on all the air vents
  • Deal with any water buildupas soon as possible(moisture attracts pests)
  • Maintain the area around the outdoor HVAC unit clean
  • Seal any gaps in the heating and cooling system
  • Regularly inspect the air vents for any signs of an infestation

Read: Can An Attic Be Used To Vent A Portable Air Conditioner?

Air Filter Slamming Against the Vent

If you have a return air vent on the ceiling, then you might hear some noises after the HVAC system gets switched off as the air filter inside that vent gets sucked up to the top of the enclosure and then released.

This can happen if you have a high-efficiency air filter or an undersized return duct.

When the system turns off and stops pulling the air in, the filter will simply get dropped onto the vent and that will cause a slamming sound.

Final Thoughts

The chances are high that you hear noises from your air vent even when the HVAC system is off because there is a malfunction that is keeping the equipment on. To check that, you can turn the whole system off at the breaker. If the noise did not go away, then it looks like you might be dealing with an infestation or with leaky ducts located in a drafty space.  

Leave a Comment