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

[old Chevy guy]

Could it be the fan hitting something? maybe a sloppy motor mount.Dont get why it would go away after shutting it off no matter what it was.

[AirSpeed]

Originally Posted by old Chevy guy

Could it be the fan hitting something? maybe a sloppy motor mount.Dont get why it would go away after shutting it off no matter what it was.

I can't figure out why shutting it off makes it go away either. The fan clears the shroud and the motor mounts are good. I can't find any interference anywhere. If it didn't go away once I shut off the engine and if it matched the engine speed I would swear its a rod knocking, I've had a couple engines with a bad rod bearing sound just like it but this just doesn't match the engine speed and it goes away. I drove it around yesterday trying to get it to happen again but no luck (or maybe it's good luck!) I'm sure it will happen again when I least expect it.
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