The reasons can range from your circuit breaker tripping, blowing a fuse, clogged air filter, dirty evaporator coils or condenser lines (on your outdoor unit), refrigerant leak, motor, thermostat failure, or there could be a wiring issue.
Check to make sure your thermostat is set to “AUTO”. The next thing to check is your air filter–if it’s dirty, change it. If these are not an issue, the problem may be serious. It is likely a professional will need to be called in.
First, you need to identify if the noise is coming from the indoor or the outdoor component of your AC unit. If indoor, there may be something obstructing proper airflow. If outdoors, several underlying issues may be the cause, such as too much internal pressure in the AC unit, a refrigerant leak, or an electrical issue