You're going to hear the hiss of the AC when using it, it's the valve opening to let more refrigerant into the evaporator. The higher the temperature difference (ambient air temperature) and the faster the blower is going the more often the valve will open. When the valve opens you'll hear a hissing sound and once the pressure equalizes enough the valve closes again. As the air going through the evaporator cools off the pressure on the evaporator side of the valve will change enough to allow the valve to open again to let more refrigerant in. This is perfectly normal as far as I am aware.
If you're concerned about it then the first thing to do is check the temperature of the air coming out of the vents with the AC on full blast on a moderate day (70 degrees Fahrenheit or about 21C). If I recall correctly it should be around 45F degrees or so (~7C to 8C). If it's much warmer than that then there is some issue, either the charge of refrigerant is too low or too high, or several other reasons.
Another thing to check is the charge level of the refrigerant. You'll either need a dual gauge set specifically meant for AC, or just take it to a shop to have them check the levels. You need to check both the high and low pressure sides to know if the charge level is correct.
But if it feels like the AC is blowing cold air properly then it's probably fine, it doesn't take much for the AC to stop cooling air enough to be noticeable if something is wrong with the system. We just have a noisy AC in the H3's.