Hello guys I am new to this forum and I have a very unique problem that has had all mechanics frazzled. So here is the story, around some time in the summer my car made a hissing noise like so pressure being relieved from the car when my a/c was turned on. Everytime this sound comes on my a/c stops working and hot air comes out of the vents. My main poblem is that my a/c makes these noises and when I drive in the highway or high speeds my a/c stops working and if I slow down and drive in the city it goes back to being cold but with occasionaly hissing noises and sometimes smoke coming out of the vents. I have changed the a/c thermal switch, and even the compressor but still the same issue even with the new compressor. I even emptied and changed the freon. Any feedback is helpful.