At idle my wife's 93 Suburban (350) has developed this high pitched whistle. I cannot locate the source. While accellerating it goes away. While coasting it comes back. If I feather the gas I can change the pitch slightly. Originally I was thinking the IAC valve was the culprit since it was only doing it at idle and near idle RPMs. I took it to AUTOZONE and found a Chevy guy there. Originally I was going to just buy the IAC, but I decided to ask for another opinion. His opinion was that the IAC would make more of a sucking sound than a whistle because air is goig in, not out. I agreed. He thought it was the alternator, so I removed the alternator and the belt. Started the truck and the sound was still there, no change. It's very hard to pinpoint because it's so loud. It actually makes me start to feel sick in the stomach when I try to listen too long while I poke around. I also want my neighbors to like me and therefore can't just let it squeal out front for 20 minutes while I poke around. We also tried spraying starter fluid all around figuring it would stumble if there was a vacuum leak. Not even a sputter. Also, it idles perfectly. I put it all back together in the parking lot (yeah, I was THAT GUY) in the rain. One last parting suggestion from the parts guy was that the bearings in the distributor were going out. Don't know about that one. Any thoughts?