Loose or improperly secured duct work within the home can vibrate with the operation of the air-conditioning unit and cause various humming and rattling noises. The duct work can either be hard and straight or flexible and distributes cold air to rooms in the home. You can access the duct work through the attic or crawl space and secure it to the structure of the house with straps or duct tape to eliminate some of the vibrations, if not all.
The air conditioner’s internal compressor is the component that activates the cooling element and produces cold air. Sometimes, when the compressor kicks in, it emits a low, humming noise, which could produce vibrations throughout the unit. While some compressor vibration is normal to a degree, there could be something wrong with the compressor if it causes the whole unit to shake. You should contact a technician to take a look at the unit if you suspect the compressor is malfunctioning.
If the condenser fan component is blocked or jammed, the fan motor will continue to run, which will cause a vibration or humming noise and possibly break the fan. The condenser fan pushes the cold air from the unit into the home. Without the fan to move the cold air out of the unit, it will freeze up. If you hear a noise and vibration while the unit is in operation but no cold air enters the home, the fan might be blocked. Turn off electricity to the unit at the circuit breaker immediately to avoid damaging the condenser fan then inspect the fan compartment for debris, such as sticks or plastic bags that might be wrapped around the axle.
A window-mounted air conditioner that’s improperly installed can cause the window frame to vibrate. Not only will the frame vibrate, the unit might not be secured tightly enough and could fall out of the window. To stop the vibrations from the window unit, you should take it out of the window frame then apply weather sealing strips to the framework before putting the unit back in. The weather sealing pads the unit against the metal frame and prevents noise and air leaks.