A little bit of vibration from the outside portion of your AC unit isn't usually a concern, but major shaking indicates that there is a problem that needs to be addressed quickly. Otherwise, the damage can become worse if repairs aren't completed.
1. Failing Compressor
If the shaking tends to be worse in the first minutes after the AC starts running, the issue is often a failing compressor motor. This is called a hard start, often caused by an electrical failure in the compressor unit. Your repair tech may be able to repair the issue, but often it means that it is time to replace the compressor.
2. Failed Fan Bearings
The AC fan is equipped with bearings, which allow it to move freely and smoothly. Over time, these bearings can lose lubrication and become jammed, which prevents the smooth movement of the fan. The result is a shaking, out-of-balance fan assembly. Your repair tech can remove the old bearing and repack the fan with new bearings. Then, the unit must be fully lubricated to prevent future shaking.
3. Dirt and Debris
Dead leaves, twigs, fur, and other types of debris can get sucked into the AC unit. These items can clog up or wrap around the fan assembly, causing it to turn poorly or out of balance. When a dirty and clogged fan begins spinning quickly, it can cause the entire AC unit to shake. Often there will also be a whining noise due to the friction of the dirt in the assembly. Repair often requires little more than cleaning out the unit and addressing fan assembly.
4. Worn Out Pads
Between the AC unit and the foundation are anti-vibration pads. An AC will always vibrate somewhat, so these pads are placed to absorb that vibration so the unit doesn't shake or make noise. Pads can wear out over time. In fact, sometimes units are installed and the pad placement is overlooked or not placed in the correct location. Replacing the anti-vibration pads will usually solve the problem.
5. Weak Foundation
An air conditioner should be installed on a concrete foundation. In some cases, the unit gets knocked askew so that it is no longer seated levelly on the foundation. This can occur for many reasons, such as snow sliding off a roof and hitting the unit, or knocking against the unit with heavy lawn equipment. The unit must be reseated properly onto the foundation and leveled in order to stop the shaking.
Contact an AC repair service if your unit is shaking and vibrating during operation. An AC repair service can provide more information.