LOUD rattle when engine starts

[Papablunt]

Pull the driver side valve cover and check everything under there. You know the drill.... Rubber hose to your ear on each rocker.

Maybe you already have and I missed it, but check for exhaust leaks. Soapy water is a good method. Here's a pretty interesting method of checking the whole system:

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[wezmykat]

It almost sounds like your valves
check out this video, i did like he did, adjustment with engine running and oil flying everywhere, maybe one of yours is loose, won't cost anything to try.

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[Keeper]

I had a bad lifter/bent push rod on my L29 that sounded like that. It also acted like you are describing. I limped it a good 10 miles home from the dump, where it started. I'd be suspicious of valvetrain also.

 

[81nascar]

You soooooooo! have an internal problem.. First just to be sure take the belt off, fire it up see if noise is still there ... that will rule out any accessories.. It really sounds like a pushrod banging away though. Take the valve covers of to see if something is broken or has major tolerance.. It's not plugs and wires, egr or any of that.. You got friction going on there. None of that stuff would cause that. You may be able to use a really long screwdriver and put your ear to it and pinpoint where it's coming from.. Just touch the engine with the tip on different spots..

 

[OrgasmicLarry69]

You soooooooo! have an internal problem.. First just to be sure take the belt off, fire it up see if noise is still there ... that will rule out any accessories.. It really sounds like a pushrod banging away though. Take the valve covers of to see if something is broken or has major tolerance.. It's not plugs and wires, egr or any of that.. You got friction going on there. None of that stuff would cause that. You may be able to use a really long screwdriver and put your ear to it and pinpoint where it's coming from.. Just touch the engine with the tip on different spots..

I ended up taking the truck to a shop
So far the mechanic told me that all the pushrods, rockers, lifters, were pulled and inspected and none were found bent, damaged, or destroyed.
And hadn’t found any metal shavings
At this point in time, the issue hasn’t been found.

 

[Jeff Janssen]

I keep coming back to your comment in the original post:

"It doesn’t do it when the starters engaged (I pulled the fuel pump relay and it didn’t do it the whole time, just when it was idling)"

The reason for the lack of noise when not firing but just cranking is still unexplained... if it were valve train related it seems loud enough that you would hear it even when driven by a starter alone?

For what it's worth, I had a noise similar (though not exactly like this) a long time ago (on a different 6cyl engine). It also only did it when running. Turned out the spark plug had failed all on it's own and dropped a healthy chunk of the porcelyn insulator into the cylinder. It bounced around there for a while (about 20 minutes of run time) until it was spat out while the mechanic was diagnosing it. We never did anything more after it was ejected and the noise went away. Drove it for 100,000 Km's after that with no problems.

 

[Hipster]

I ended up taking the truck to a shop
So far the mechanic told me that all the pushrods, rockers, lifters, were pulled and inspected and none were found bent, damaged, or destroyed.
And hadn’t found any metal shavings
At this point in time, the issue hasn’t been found.

I'm not sure why the mechanic tore down the valve train if he didn't find something loose. With the level of noise you're having if it was a valvetrain issue something would have almost certainly been loose and evident once the valve cover was off.

It doesn't really sound like a valve train noise to me, More like a fragged A/C compressor or something in the rotating assembly.