I believe that not all hissing is abnormal...the cycling of the compressor as it pressurizes will cause that sound. Mine always had, regardless of freon levels...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car