OK here goes. I recently rebuilt the top half of my 1992 Chevy c-1500 pickup with the 5.7 in it. Had my head gasket go so it had to be done. Now once it was all said and done I all the sudden had weird vibration noise when the AC was on and the rpms passed from about 1100 to 1300. This noise is only happening under those circumstances. AC off and no noise at all at any rpm. Now I read extensively on several forums and all of which said the AC compressor was either going bad or the clutch bearing was. Now with that in mind I went ahead and replaced the AC compressor with a new one and still have the same exact noise. I decided then to try out replacing the belt tensioner and idler pulley so I did and same noise at same rpms with AC running. WTH. Don't want to keep replacing things until I'm broke. All that's left is the water pump, alternator and power steering pump. Fluid is not low in the PS pump either. Now i did notice a very small amount of play in the water pump pulley but the sound definately is its loudest right by the AC compressor. Any ideas? I'm sure I have the ghost in the machine and hope someone has exercised this demon.