Air conditioners, when they are functioning optimally, are pretty quiet. There can be the occasional noise when they complete a cycle but if they suddenly start squealing, hissing, buzzing, rattling, clicking, or banging - you get the picture - then it is time to do some investigation. Rattling or banging could mean something has come loose, while buzzing often means you have an electrical issue. Any kind of persistent noise is a cry for help from your air conditioning unit. Turn off your AC and call an HVAC professional to come out and assess your system for issues.