A/C Interior Hissing Noise

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As the liquid refrigerant evaporates in the cooling coil under the dash (appropriately called the "evaporator"), thus absorbing heat, there will be a slight hissing sound emitted. Perhaps previously your air conditioner was low on refrigerant, so less liquid refrigerant and more gaseous was present, reducing the hissing sound. You already say the A/C cools better than before, which supports this hypothesis.

I agree that the evaporating refrigerant would produce a slight, constant hissing sound. However, what I have is a somewhat loud hissing sound that comes and goes frequently while driving. It seems to me this might mean the system is actually low on refrigerant so sometimes air is rushing through the evaporator nozzle (the hissing sound) and at other times there is liquid coming through like it is supposed to which does not make an audible sound. If you compare this situation to the sound a faucet makes when you have air in your pipes (say, after turning the water off for a winter vacation) it is similar. There is little to no sound when water is coming out but very loud sounds when air is rushing much more quickly through the same size opening.

I didn't see the original post, but if that is what you think, then I suggest trying to correlate the sound you hear to the on/off cycling of your a/c compressor. That would very easy to do, but I will leave how you do it up to you. Then at least you'll know why it comes and goes.

I had the same problem with my 1998 Grand Voyager. Took it to the local dealer who had worked on the "gates" behind the dash area near the radio prior. After 4 hours they came back and said they could not find anything but they re gassed it and said there would be no charge for time incurred ? Strange but true. I asked twice but could not get a straight answer. But i think it could be a joint leak behind the dash area. Maybe disturbed by them ?