Bizzarre intermittent sound

[AirSpeed]

Recently my 72 GMC 4X4 with 350/350 has started making a strange sound. It's happened four times in the last three weeks. It always starts after I start the truck, it doesn't matter if the engine is warm or cold, as I accelerate I hear a sound coming from the front, it sounds like a rod knocking but the sound doesnt sound like it matches the engine speed like a rod knocking, if I take it out of gear while rolling and press the gas it doesn't make the sound, once I put it in gear and press the gas it starts again. To get it to stop I have to pull over, shut off the engine and start it again. There's no vibration and no power loss. My drive lines both have new u joints. It's bugging the crap out of me! Any ideas?
Posted via Mobile Device
Posted via Mobile Device