Whether your air conditioner is brand new or nearing the end of its lifespan, unexpected noises are a well-justified concern. This is especially true when these sounds occur after your AC unit has already been turned off. There is usually a clear reason why the cooling system in your Richmond, VA, home is hissing, clanging, humming, or rattling, and knowing this reason will help you identify and resolve the underlying problem. The following are eight common sounds that air conditioners make after they’ve powered off and their cause.

1. Banging Noises at the Building Interior

Your central air conditioner has an indoor air handler/evaporator unit and an outdoor condenser/compressor unit. If you hear banging from inside your home, it’s likely coming from the air handler.

Not surprisingly, the most likely cause of this noise is your air filter. Air filters lie at the heart of countless AC problems. When these components are dirty and weighed down by thick accumulations of lint, dust, and other grime, changes in air pressure at the time of shutdown could cause them to slam against the grille. Banging noises caused by AC filters can also result from improper filter sizing or incorrect filter installation. To troubleshoot, make sure that your AC filter is clean, installed correctly, and not too restrictive.

If your air filter isn’t the problem, loud banging noises from inside the home could indicate a far more serious issue. They might mean that the system’s blower wheel has come loose or is completely detached and is now forcefully bumping into other system components.

2. Slamming Noises Within Your HVAC Ductwork

Loud, jarring sounds just after your air conditioner turns off aren’t guaranteed to originate from your cooling equipment. Instead, these noises may be entirely duct-related. If you have HVAC air duct dampers for central HVAC zoning or manual customization of air delivery, the slamming noises that you hear are probably just your duct dampers swinging shut as airflow ceases. Slight adjustments by a qualified HVAC technician could help mute these noises.

Even if you don’t have dampers installed, loud slamming sounds could still be related to the ducts. Like many other building materials, HVAC air ducts expand and contract due to sudden and significant temperature or pressure changes. These movements typically occur just after the unit shuts down, frequently resulting in loud clanging, slamming, or banging sounds.

3. Banging Noises at the Outdoor Condenser Unit

Located in your back or side yard, your AC condenser houses the system’s compressor. This is easily the most costly component in your air cooling system, and if your air conditioner’s compressor fails, it’s likely time for AC replacement.

When correctly sized, professionally installed, and well-maintained, most residential air conditioners last between 15 and 20 years. If your air conditioner has already passed its 10-year mark and you hear loud banging sounds coming from the condenser at the exterior of your home, these noises could indicate compressor failure. As compressors age, they gradually become imbalanced. This causes their refrigerant tanks to bang against their sidewalls during and just after operation.

4. Rattling Sounds at the AC Condenser

If the noise coming from your AC condenser isn’t loud and heavy, it could be the result of fan problems. Clanging, whirring, and clicking sounds are often an indication of loose components within the fan, bent or displaced fan blades, or obstructions. You might hear these noises during operation and when the air conditioner is actively shutting down. However, they’re unlikely to occur after the AC has already been off for several minutes.

5. Buzzing When the Air Conditioner Is Completely Off

Most of the sounds that you might hear just after your air conditioner is turned off are the result of expanding and contracting components, changes in heat and pressure, loose components, or gradual decreases in the speed of moving parts. In short, they’ll quickly abate after conditions change. These are noises that might last for two to 10 minutes after the last cooling cycle has ended.

However, buzzing noises can occur long after your air conditioner has stopped running. In most cases, these are the direct result of wiring or other electrical issues. For instance, it might be that the air conditioner’s relay switch has stopped functioning normally. The relay switch controls the flow of electricity to the AC compressor. When it isn’t working, power is affected. This causes a persistent buzzing or humming sound.

When compressors fail, this can also affect how voltage flows to and through these units. In this instance, the buzzing is most likely to occur when the blower is functioning but the fan cannot turn. Whenever electrical problems are suspected, it’s important to keep the air conditioner off and schedule repairs. Attempting to run an air conditioner with electrical issues is a major fire hazard. It can also result in the need for AC replacement rather than a relatively low-cost fix.

6. Hissing and Trickling Noises

The refrigerant inside air conditioners is under tremendous pressure during operation. The change in this pressure often leads to hissing noises just after cooling systems shut down. Although many brand-new air conditioners offer whisper-quiet operation both indoors and out, some AC models are known to produce faint hissing sounds every time they’re turned off. However, if hissing is especially loud and prolonged and is paired with trickling or dripping noises, you may have a refrigerant leak or problems along your cooling system’s condensate line. Much like electrical problems, refrigerant leaks, and condensate line repairs should only be addressed by professional HVAC technicians.

7. Clicking and Ticking Sounds After ACs Are Turned Off

Buzzing noises aren’t the only sounds that can indicate electrical problems. If you hear clicking or ticking inside or outside of your home just at start-up and right after your AC turns off, these may be a sign of loose wiring, faulty connections, or impending compressor failure. Although air conditioners will often continue working for weeks or even months after clicking noises start, it’s best to turn them off as soon as they appear. Avoid turning them back on until the underlying problem has been resolved.

8. Scratching, Clawing, and Scuttling Noises

Sometimes the loudest and most alarming sounds from air conditioners aren’t due to problems within the units themselves. Instead, they’re caused by pests that have found their way into HVAC air ducts or outdoor condenser units. Whenever there’s a viable point of ingress, there’s always the risk of having insects, rats, mice, and other pests enter these spaces to capitalize on their shelter, warmth, and easy access to indoor areas that they provide. If your air conditioner is off but there are frequent bumping, clawing, scuttling, or scratching noises in your ducting, you should call an HVAC company or a licensed pest control service right away.

Contact Us

Paying attention to strange AC sounds could help you identify developing problems before they’ve already spiraled out of control. Whether changing your air filter or scheduling professional AC service, taking timely action to identify and resolve the cause of these noises will save you a considerable amount of money.

Since 1984, Davis & Green has proudly served residents of Richmond, VA and the surrounding areas. We offer heating, cooling, plumbing, and electrical services. We also provide air quality testing and preventative maintenance plans. If your air conditioner makes strange noises after it shuts down, contact Davis & Green to schedule a visit.

Davis & Green

company icon