1990 Suburban high idle/whistle from IAC

[SoCalSlaughter]

I've had this problem with my Suburban for as long as I can remember. First off, when the truck is started cold, it runs a bit rough, once it warms up it runs much better. It will idle fine, has power, no engine light. But after about 10 minutes of driving, I notice on acceleration, after I let go of the gas, I can hear a whistle. When I come to a stop light/sign, the trucks idle is high, about 1,200rpm. If I put the truck into park it will idle at 1600/1800 rpms. Once the truck has been turned off and started back up the idle will be fine until I start driving again. I've compression tested all 8 cylinders, which where all between 147 to 151. Ive replaced the throttle body gasket, replaced the IAC motor and have tried switching it out from a friends 1990 Suburban that was known working, replaced O2 sensor, coolant temp sensor, throttle body was rebuilt with new injectors, all normal maintenance has been done. Ive replaced the positive and negative batter cables, knock sensor has been replaced, the ECM has also been swapped out to rule out a faulty ECM. The wiring harness has been pulled out of the truck and checked for bear marks and/or bear wires. The only code the truck has gotten recently was a code 42 (ignition control module). That code hasn't been set in about 2 months. All vacuum lines have been replaced. The truck runs awesome when its not in this condition. I am now at a loss of what to do or check for. It seems I've gone through the engine bay 30 times. The truck was also able to pass Calif smog running like this. But I think it may be affecting my mpg's. The truck is a 1990 Suburban V2500 5.7 TBI 4X4 TH400. Thanks guys!!!