Rattle on first startup

130k miles V6 not sure on the tranny. It makes the noise once the idle settles at cold start then eventually tapers off to a more faint sound. It’s really not so much a rattle more an echo sound. If I get right down at the cat it sound like it’s right there. The one rattle sound at start up that’s only happened 2x sounds like it could be same spot but hard to tell since it was so random. Would running iridium vs copper plugs cause an issue ?

 

I second this. I had a seized block so I poured marvel mystery oil down the spark plug tubes and let it sit for a week. Turned over effortlessly after soaking in MMO. I would pull the valve covers and squirt some MMO around the valve shims then let it sit a day or two. I’d say pull the cams and remove the shims, then drip some MMO directly on the valves but might be more invasive than you’d want to go initially.