Mystery rattle on startup

[Matthew Jeschke]

I cannot figure out what this is rattling. I looked under the truck to see if anything is in close contact. I have some piston slap but I don't think this is that? I also fixed all my sticky lifters. I've been idling the truck (but noise isn't really there at idle) and using a mechanic's stethoscope. I've probed:

the transmission,
bell housing (took inspection cover off to see if anything was loose in there),
starter
each cylinder by exhaust manifold,
all the injectors
Above the lifters on the valley
the valve covers
all the accessories

I cannot seem to find anything odd?

Maybe a collapsed lifter that struggles to build pressure? Or a bracket that's loose? Anybody have any theories as to what this noise is. I made a video of it below:

https://drive.google.com/file/d/1hNHezXNqEGVS4NseI56iT1pAPj74YCr3/view?usp=sharing

 

[Fless]

Starter not disengaging completely?

 

[exp500]

Flex plate loose/cracked? Exhaust donuts leak/ slip/rattle. Heat shield rattle? Start putting your hands on everything see if it moves.

 

[Matthew Jeschke]

I turned the flex plate manually with a flathead screwdriver. Looks perfect at least through the inspection cover.

I thought maybe starter as I just replaced that but rattle doesnt seem to perfectly correspond with starter operation.

I think I found my issue though. I got under the truck and started banging around. The driver side cat is coming appart inside. It rattles whenever I tap it with a mallot.

I guess they don't always last 240,000 miles haha Seems to work for now but I may need to replace it eventually.

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[OR VietVet]

When cats come apart inside they can make that noise but they also tend to still rattle when idling in gear. Does it do that. Before you brought up the cat problem I was going to say maybe a belt tensioner rattle when engine first starts when the spring tension has diminished over time. That crank and engine start is ******* those tensioners over time.

 

[Matthew Jeschke]

Yeah it is more subtle when idling. I used a mechanics stethescope to listen to the exhaust above the cat on it and below it.

Noise was worst on the cat. Virtually nothing above it or below it.

I wonder if I could just put a pipe there. I would somehow have to disable the rear O2 sensors.

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[exp500]

Try Spark plug Anti Foulers on rear O2 sensors, sometimes they fool the puter. For 6 bucks it's worth a try.

 

[Matthew Jeschke]

Bad ass, I will have to try that out, if/when I ever get to deleting the cats haha

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