Newer HVAC units tend to run quietly. Older versions may emit sounds when the system starts up or when it's running, and it’s considered quite normal. However, if the noise is loud enough to interrupt you from binge-watching your latest TV shows, the problem may be more serious. If your AC unit is communicating through a series of hissing and whistles, it may be the right time to call an HVAC professional for immediate examination and rectification. Here are common sounds the technician may deduce.