Have a 2005 Grand Caravan, a few days ago it started to blow warm air, I can hear ac compressor clutch kick in and also a hissing noise coming from behind glove comparment, not the blower, but it sounds like when air is coming out from a tire...hisssing....when i turn ac off the hissing stays on for a few seconds then quits...any ideas? I haven't checked anything yet....dont know where to start.

 

Sounds like a vacuum leak for your blend doors. They're not getting vacuum, causing it not to switch over from heat to ac.

Problem is it will probably require pulling the dash to fix.