while running the air cond. i hear a click and it starts blowing hot air

Yes it was checked a few months ago,i know that doesnt mean its ok now but i am pretty sure thats not it i keep the motor area clean so i can get a heads up on any leaks and the click or thunk noise it makes when it does it comes from the area around the blower not the air compressor some times it runs cold for 20 min. and at other times its less then a min.and it always makes that thunking sound under the dash on the passenger side .is there a vacuum valve or something like that in that area?