I have refrigerant lines that have a right angle in them and they too make a bit of a hissing sound. There could be a restriction in the line somewhere. Not enough to cause a problem with operating but just enough to make a bit of a noise, Maybe a bit of a bend or an angle in the pipe within the wall. As for the dampness, Not sure but your coil should be draining the condensate off so after running a while the air will dry somewhat. Could be only damp when it first starts.