I just installed a new 3.8 engine from a junk yard with 700 mile on it and have a real strange sqealing noise almost like a rollerbearing or belt squeal, which I have done several checks such as removing belt to see if any tension pully or alt or water pump ect. making the noise but still continues. I have gotten under engine and noise sounds like it is coming from knock sensor area or buy ac compressor or the center of the engine block,this is where it is the loudest. I now a bit about motors and this is the strangest thing I have ever heard the engine runs great have put 700 more miles on it before this started, it only happens when engine is warmed up to operating temp and fades in and out at above 1000 rpm, trans is also been rebuilt at GM dealer under warranty. This isa head scratcher. Please help I am going crazy with this??? I just changed harmonic balancer as well still making same noise.

Strange, so you are sure it's not any belts, the way I check, is hose them down with WD40 it will cause the belt to slip not turn the pullys untill the friction of slipping burns the WD off, usualt 20 secconds or so. if it's not belts, then I'd hate to think of whatelse it could be. maybe your crank berings? Does it happen at idle or in gear?