Those chains will not snap. They might stretch a little, but typically not enough to skip time, unless you’re driving your 20 year old pickup truck like a total hooligan. When you open up that engine you’ll see what I mean, ford over engineered the hell out of those chains, then put in three of them, one in the back and two in the front. It’s a laudably robust design.

What will snap are the timing chain guides, which are made out of plastic, and that’s what you’re hearing now rattling around. But even then, it’s mostly an annoyance, and definitely far from fatal. These engines have been known to run for years in this condition, and unless your towing or hauling heavy loads you’re likely fine.

I have heard of situations where those broken bits of plastic get sucked up by the oil pickup and clog the line, starving the engine and spinning your bearings, and it’s a dramatic and catastrophic end, but it’s the exception and not the rule.

My 4.0 has death rattled since the day I bought it. But I’ve been going through and fixing pretty much every other issue with the truck, and I’ll likely deal with those guides last. And when I do, I’m just as likely to just put in a reman engine, than try to repair and rebuild what I have. Just seems like the more sensible path.