Hissing sound when AC is on

When my AC is turned on by manual or automatic, I hear a hissing sort of sound. WHen the AC is off the sound goes away. It has nothing to do with the fan, because the sound gets louder as i dial the temp down and not the fan speed. Could this possibbly be to low R134a? I put my hands on the compressor high pressure and low pressure. Felt hot on one, and cool on the other, but not as cold as it should be i think

Please help

You can have it checked to see if it needs recharged, my LHS does it too but the AC works fine and blows nice and cold, I'm thinking it's just the sound of the refrigerant passing through the evaporator coil in the car.

Well, I have never heard this before in the 2 years i have owned the car. No cold air is blowing which makes me think it is an r134 issue. I was thinking that it could also be the vacume things that switch when AC is turnd on in the vents. because when the AC is off he noise goes away, and when turned on it may be a partiall opened vacume