I'm working on a 1999 5.4L Triton engine. It had a rattle on startup that developed into a knock. The #3/#7 rod bearings burned on the crankshaft. Either before or after this the #3 and #5 crank bearings were spinning and oil starved the two rod journals. Don't ignore engine rattles. If rattle is in the top-end, check for worn cam or rockers. If in the bottom check for burned bearings. It's cheaper to fix than to replace an engine. This one only had 98,000 on it.