Skip Jarvis

I had the same problem, Intermittent whistling when on A/C. I noticed the whistling would sound when the compressor engaged, but then fade out while the compressor continued engaged, Whistle, no whistle, whistle, no whistle. I took it to a repair shop where they said it was down a little on freon, that they changed my fan clutch, and charged the system back up. They admitted that they had no idea what the whistle was. Yes, I listened to it on the way home, very disappointed. I began to notice that the air from the vents would quickly cool when the compressor engaged and while it was making the whistling sound, but when the whistling stopped, you could feel the temperature at the vent rising quickly. It began to fail to be able to cool the cab. I decided to take it to my local dealer, Wes Haney Chevrolet in Live Oak, Florida. The service manager drove it and admitted he had never heard a whistle like that before. Here is what they did. They evacuated the entire system and checked it for leaks. None found. They then recharged it by the weight of the freon, not the pressure in the system. They advised that it took about a half pound more of freon than it had in it. WHISTLE IS NOW GONE!!!! A/C blowing cold. The truck has 176,000 miles on it and they advised that the compressor is getting along in years, but that they could not justify replacing it yet while everything is working fine. My truck is a 2004 Chevrolet Silverado Z71 with 176,000 miles. Maybe this information might solve your problem.