Help! My Air Conditioner Compressor makes a Buzzing Noise Instead of Starting

Help! My Air Conditioner Compressor makes a Buzzing Noise Instead of Starting
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Air conditioning in your home is a necessity in many parts of the country and the world. Noisy compressors can signal relatively minor problems with easy fixes or they can be the harbinger of a compressor that’s about to fail. The compressor is the portion of your air conditioner that pressurizes the refrigerant in the system, causing the refrigerant to become cold. There are a few different reasons why your compressor may be making buzzing noises.

Faulty Disconnect/Fuse

Cutler Hammer AC Disconnect

Square D AC Disconnect

The compressor in your air conditioner needs a specific amount of voltage at a specific amperage in order to start running properly. If for any reason it doesn’t receive the required amount of power, it may exhibit a buzzing sound from the compressor, as the compressor tries to start. Access the system electrical disconnect and inspect it for corroded or worn out contacts. Cycle the power disconnect three or four times to ensure that the contact being made is sufficient. This may entail moving an actuator switch up and down a few times, or inserting and removing a fuse holder. If there are fuses, test them for continuity or resistance with a multimeter.

Missing/Damaged Isolation Feet

Every compressor is mounted to the base of the unit with bolts that have rubber feet called isolation feet. If the unit is relatively old, these feet can harden and crack. Over time, they can crack enough that they can disintegrate. Once this happens, the compressors becomes unbalanced, which can cause a number of problems, many of which will be exhibited by the buzzing sound you’re hearing.

To troubleshoot this, disconnect the unit from power and remove the top cover, using the proper tool (usually either a Phillips screwdriver or a 3/8 or ¼ inch nut driver). With the cover off, inspect the four points where the compressor mounts to the base of the air conditioner and ensure that all four rubber feet are in place and in good condition. If they are cracked severely or missing, replace them. This will require using the correct tool to remove the screws or bolts that secure the mounts to the base and sliding the isolation feet off the mounting ears of the compressor.

Loose Parts Vibrating

The fan can create a vibration sound if it’

Loose parts vibrating against each other can also cause buzzing noises in your air conditioning compressor unit. Disconnect the power and take the cover off. Shake all of the internal parts to ensure that they are properly secured. Check to make sure all nuts, bolts, and connections are tight. Additionally, parts that touch each other should either have clamps or isolators to keep them from rubbing. Check the bottom side of the cover to make sure it isn’t bent and being hit by the condenser fan. Also verify that the screws holding the fan guard to the cover are tight.

Failing Contactor

Typical Contactor

Turn the power to your air conditioner off and remove the electrical panel cover from the outside of the unit. Locate the contactor. It may look like the contactor in the picture or it may be similar, but have a metal bar across the top. If you have a multimeter, turn the air conditioner on and measure the voltage drop across the contactor. There should be no to minimal voltage drop across it. If you don’t have a meter, find a piece of wood about twelve inches long and one inch square. Turn the air conditioner power back on and watch and listen for the contactor to buzz. Press the moving part of the contactor against the body with the piece of wood. If the buzzing goes away or diminishes, it will need to be replaced. Disconnect power to the unit again, label the wires going to the contactor and remove them. Remove the screws or bolts securing the contactor in the housing and remove the part. Air conditioning and appliance supply stores, as well as some large hardware/home improvement stores, should carry replacements. These should only be replaced with identical parts.

Failing Run or Hard Start Capacitor

Typical Capacitors

Turn the power off for the air conditioner and remove the cover and the electrical panel cover. Locate the capacitor that provides an extra surge of power to start the compressor. Inspect it to ensure that it hasn’t exploded or otherwise leaked electrolyte. Make sure the wires to it are properly connected and not broken or frayed. If you don’t have a meter capable of testing capacitors, disconnect the wires after labeling them to indicate where they go (take a picture also, it helps) and take it to an appliance supply store and have them test it. If everything else has checked out, this will most likely be the problem. You may have two capacitors for the condenser in an older unit. One is the run capacitor while the other is for hard starting compressors. Both should be replaced.


The author is a certified electrician.

Contactor and capacitor images courtesy Wiki Commons

All other images provided by author.