Its 100 degrees, 50% humidity and your AC just stopped working! Now you face the miserable wait for the technician while the temperature in your home creeps towards "uninhabitable."
Ideally, you would call a technician before your AC stopped - so, how can you know in advance? Sounds from the unit or compressor are one of the indicators that something is wrong and you might be about to lose your cooling system. If your AC unit starts squealing or squeaking, you should do something about it quickly.
Noise from the compressor unit indicates that there is a problem. Rattling means something is coming loose. Loud banging or clanking means you have a serious problem and should turn the unit off before more damage is done. A sound from the blower usually means something is caught on the blades and should be checked because it lowers the motor's efficiency.
In general, any noise from the compressor that you have not heard before is cause for concern. You should become familiar with any sounds your compressor unit makes during normal operation so you know if something is wrong. If you hear an unusual noise, call a qualified technician to inspect your system.