An AC making loud or unusual noises is usually narrowed down by the type of sound. Start with what the noise sounds like.
Buzzing or humming from the outdoor unit with no fan rotation is almost always a failed capacitor. The compressor and condenser-fan motor are trying to start, can't, and pull locked-rotor current until they trip on thermal overload. Shut the system off immediately and call for service — running a locked compressor for more than a minute or two damages the windings. Capacitor replacement runs $150 to $300.
Screeching or squealing is typically a bearing — either the condenser-fan motor bearings or the indoor blower-motor bearings. Bearing failure is progressive: the noise gets worse over days to weeks until the motor seizes. Motor replacement runs $300 to $600 for most residential units.
Repeated clicking during startup is the contactor chattering. The contactor is the high-voltage relay that feeds the compressor; when its coil or contacts fail, it rapidly opens and closes instead of latching. A $50 contactor and 30 minutes of labor fix it.
Banging or rattling is mechanical — a loose fan blade, a failing compressor mount, or a sheet-metal panel that has lost a screw. The outdoor unit is designed to be hand-tight on most panels; panels that have backed off at the screws generate a surprising amount of noise.
A loud "hissing" sound from the refrigerant lines is different and warrants immediate shutoff — that's refrigerant escaping through a rupture, and continued operation damages the compressor.
Some noise is normal. An outdoor condensing unit at startup pressurizes the refrigerant circuit quickly; a brief ramp-up sound in the first few seconds of compressor operation is expected. Compressor run noise above 75 dB at the unit is not expected and deserves a service call.