The answer to 'AC making noise' is that there are several common causes, including a worn compressor, a problem with the fan motor, or an issue with the refrigerant. Symptoms may include a grinding, squealing, or rattling noise coming from the outdoor unit. In some cases, the noise may be coming from the indoor air handler as well.
If the noise is accompanied by a loss of cooling performance, it's likely a more serious issue that will require professional repair. However, some minor noises may be due to loose parts or debris, which a homeowner could potentially address themselves. It's always best to troubleshoot the issue first before calling in an HVAC technician.