A/C equipment and duct systems are designed so you never notice have to the compressor whirring or the smooth swoosh of cool air blowing from the vents.

That is why hearing a new sound makes you perk up and pay attention.  When you notice sounds that weren’t there yesterday, there is a chance something is wrong.  If new sounds appear, begin with a little investigation to get an idea how bad the problem is.  Before you get started, let’s define a few important terms.

Your home heating and air conditioning unit will either be a “split system” or a “package unit”.  A split system has a piece of equipment inside the house installed in a closet or up in the attic.  This is the air handler. There is also a piece of equipment sitting outside called the condensing unit.  The system is split into two pieces (air handler and condensing unit), hence the name split system.  If you don’t have a split system and all of the equipment is together in one housing, this is called a package unit.  All of the components are together in one package which is where the name comes from.

Whistling

So what are the new sounds and what could be the cause?  A whistling or roaring sound from a supply air vent could mean the vent has either too much air flowing through it or the volume damper inside the vent is partially closed restricting the normal airflow.  Homeowners often close vents in empty rooms so they don’t spend money to heat or cool unoccupied space. When you close off a vent, the air that’s supposed to go into that room is forced to other areas of the house.  Shut off a lot of vents and you may push too much air into the still-open vents making them noisy. Closed vents might also whistle since most dampers don’t block the airflow completely. The whistling or roaring noise might be annoying, but closing off too many vents can inhibit airflow through the unit and cause a malfunction.  Try to limit the amount of rooms you close off to keep the system quiet and the equipment operating properly.

Rattling or Chattering

Rattling or chattering sounds are a good sign that something is loose.  Go to the outdoor condensing unit and see if a stick or other debris is hitting the condenser fan.  You can clear the sticks away and solve the noise problem right then. But if the rattling is inside the equipment itself, it could be a loose screw on the housing (an easy do-it-yourself fix), a loose condenser fan (easy to fix, but call a service technician), or a failing compressor (not easy, not cheap, call a professional).  Rattling at the indoor air handler may also be simple loose screws or a wobbly supply air fan. You may or may not feel comfortable making a repair on the air handler depending on how handy you are and where the unit is. Just be careful not to make a small problem worse by trying to fix rattles without really knowing the cause.

Bubbling or Gurgling

If your split system is low on refrigerant (refrigerant is sometimes called “Freon”) you may hear a bubbling or gurgling sound in the walls and in the refrigerant piping.  This isn’t an emergency, but low refrigerant goes hand in hand with poor performance and the system will take a long time to heat or cool the house. It’s best to schedule a service appointment to have the leak repaired and the refrigerant system refilled.  Be aware that older a/c units use outdated refrigerant that may be unavailable or extremely expensive. Don’t be surprised if you are faced with the choice or replacing the system or buying very costly refrigerant.

Squeaking

Loose fan belts or worn bearings can make a variety of squeaks and high pitched sounds.  Luckily, most of these squeaks don’t signify a pending equipment failure and can be easily fixed by a service person.  But if you hear a very loud “screech” coming from the condensing section or the package unit, the prognosis is not good.  This screech is likely the final cry of a dying compressor and, other than completely changing out the equipment, a compressor replacement is perhaps the most costly a/c repair.  If you suspect any of these sounds are from the compressor, turn off the unit right away and contact a repair company to help get things working again.

Clicking

A click from the thermostat or furnace is normal when the equipment turns off or on, but rapid fire clicking or clicking that never stops means there is some kind of electrical problem.  Bad capacitors or contactors, failing switches, maybe even a faulty thermostat are all culprits when the clicking doesn’t end. Electrical clicking problems can occur in air conditioners, heat pumps, furnaces, split systems, and package units and with each kind of system, the clicking could be something different.  Don’t let non-stop clicking go without someone determining the exact cause of the problem.

Dripping Water

The sound of dripping water in the walls requires immediate action.  This is different from the bubbling sound in the refrigerant pipes. Dripping sounds are most likely a water leak from the indoor air handler.  The drain line from the air handler to the outside might be blocked, cracked, or come apart. Every homeowner knows the consequences of water leaks in walls and ceilings: sheetrock damage, buckling floors, or mold.  If you hear dripping, turn off the a/c right away and have the drain line checked. Consider having someone test for mold in the wet areas especially if the problem has been going on for a while.

You should never notice the sound of your heating and air conditioning system.  If you hear something out of the ordinary, pay attention because unusual noises do not go away or get better with time.  Although some of the fixes are easy for a handy homeowner to tackle, many of the sounds need to be investigated by an experienced technician.  The worst thing you can do is just ignore a new squeak or rattle. Making a simple repair now could save you the cost of a major equipment replacement in the future.