2007 Yukon SLT 182k. Everything has been great. Today on lunch I turned on my A/C and heard a loud whistling sound. Seemed to be from under the hood. A/C never got cold. Clutch looks to be engaging and compressor runs constantly. Still no cold air. Blend door seems ok, because when I change the temp to 90 the air in from the vents get a lot hotter. Also, all 3 zones act the same. Any suggestions? Any common issues? I don't have pressures yet.