I still have the sound, but I may have found the cause. The line going to the cooling fan is leaking. So my theory is this: When the engine is cold the fan is not running. As the engine starts to warm up the fan comes on, but at reduced speed (hence reduce pressure). The reduce pressure combined with the leak is causing the sound. Now I'm not sure if this theory is correct. The part I don't understand is why I can feel the noise in the truck. I can feel the sound in the pedals, well really I can fell it everywhere. But once the truck is warmed up, the sound is no longer there. The noise/vibe is also proportional to the amount of pedal input. So at 25mph, no noise. I start to give it some gas and the noise is slight. More gas and the noise gets louder. If I reduce the throttle input, the noise reduces. If I come off the pedal completely, the noise goes away. This is happening regardless of MPH. Does the fan speed vary with RPM when the engine is not up to operating temp? That might explain why the noise varies with pedal input.