The part(s) or condition(s) listed below for the symptom Central air conditioner is noisy or loud are ordered from most likely to least likely to occur. Check or test each item, starting with the items at the top of the page.
The fan blade on the condenser fan motor might be defective or loose. To check the fan blade, wiggle the fan motor shaft. If the motor shaft is loose, tighten or replace the set screw. If the condenser fan motor is still making noise, replace the fan blade.
The condenser fan motor has bearings that wear out over time. When the bearings are worn out, they make loud noise. To check the bearings, try rotating the condenser fan motor by hand. If the motor does not turn smoothly, replace it.
The furnace control board sends a low voltage signal to the contactor. When the contactor receives the signal from the control board, it closes the circuit, sending voltage to the condensing unit components. If the contactor is defective, it will not provide voltage to the condensing unit. In addition, the contactor will make noise when it is defective. If the condensing unit is making noise but not cooling, the contactor has probably failed. To determine if the contactor is at fault, use a multimeter to test the contactor for continuity. If the contactor does not have continuity, replace it.
The compressor pumps refrigerant through the central air conditioning unit. When the compressor is wearing out, it generates more noise than usual. When this happens, it is not necessary to replace the compressor—compressors often last for years after they become noisy. However, if you would prefer to have a new compressor, you must have a licensed professional replace it.