I encountered a very loud and high pitch whining the other day from the engine compartment. I have a 93 C1500, 4.3L, ac, 203k miles. It begins when the engine is warmed up usually about 5-10 miles of driving. It does not occur when I am accelerating, only when I let off the gas or at idle. I have eliminated some possibilities, I removed the serpentine and started it up, still there, I used soapy water to check the hoses and from what I could tell, no leaks, saw no bubbling around the tbi or brake power booster. It sounds similar to a very loud tea kettle now that I think of it. I am at a loss, please help if you can. I need my baby for work.