I was coming from work last Tuesday night, had just gotten up to speed on the freeway, when I heard a loud knocking rattle from the engine. Checked the guages, all good, pulled over onto the shoulder,shut it off,got out to look. All fluid levels ok, started it and slowly rolled down the shoulder to home. (4 miles) It gradually got quieter, but still there. I suspected a rocker arm, so pulled VC and checked, no issues there. Now for a couple of weeks, its been getting hard to start, so checked the GPs, all 8 dead. pick up new Motorcraft's from O-reillys, come home & change them while its still warm. The first 4 come out looking ok, the next 4 all had 3/8" missing from the tips. Dang Autolites. I put them in 2 years ago when I first got the truck before I knew any better. Now I can only assume that the knocking noise I heard was one of those tips getting smashed around in the combustion chamber. The noise is gone, I guess I got lucky!