Grinding AC/Heator switcharoo

[thaprince]

Hi everyone! I checked this forum so long that I'm desperate need of some Advil! My fiance has a '98 Jimmy, and last week her heater was blowing cool air. We already got the radiator fluid replaced, but now we have a new problem...

When she turns on the AC, it blows cold air, then switches to hot, then a few moments later continues to go from cold to hot agian. Also, whether the heater/ac is on or not, there is a grinding/whining noise comming from behind her dash. It sounds like a something is trying to move, but its path is obstructede. A local mechanic said it would take 6-8 hours just to diagnose the problem! At $85/hr, I felt the boards knowledge would be a much better source! Thanks!