1999 Dodge Caravan Hissing sound while A/C on

I have recently worked on my A/C which required me and an auto mechanic friend to completely remove the dash down to the firewall in order to replace the evaporator which was corroded and leaking coolant. We replaced that, replaced all O-rings, the blower and replaced the conditioner/drier for the A/C system. The compressor was replaced in 2004. After all was said and done we recharged the system and also tested vacuum pressure. Everything was fine and it has worked great for several months. Now there is a hissing sound that travels between the front dash and the back of the van near the rear A/C blower. This van has two blowers for front and rear passengers. The A/C is not working as well as it was and I'm afraid that it either has a leak or there is some kind of pressure issue. The hissing sound is very similar to the sounds I heard when my A/C broke the first time. Anyone have any ideas on why there might be a migrating hissing sound traveling from front to back to front to back?

BTW when I turn the A/C off it takes a few seconds for the hissing sound to stop but it does stop. Blower fans on without A/C turned on does not making hissing sound.