So my sister just bought a 99 E350 with the V10. 150k miles. When she bought it it was making a knocking noise. The previous owner said it had been making the noise for awhile. The van seemed to run just fine. She just drove 1,000+ miles out to me and now I'm borrowing the van to transport some stuff around town and I'm a little worried. It's a pretty loud knock. I haven't had time to poke around the engine with a screwdriver to my ear and probably won't have time to do that for another week so I figured I'd post.

The knocking seems like it's dependent on throttle position / load. Nothing at idle. Nothing at low throttle positions (0-15%). It starts at about 15% throttle and gets worse (louder and higher frequency) up to 40% throttle. If you go past 40% throttle it disappears completely. That's the weird part to me. Give it more gas and it goes away. I could be way off on the throttle positions but you get the idea. If you want to cruise on a level road at 70mph the engine will be knocking. As soon as you go up a hill at 70 the extra throttle makes the noise disappear. It's not RPM dependent because the noise goes away instantly when you give it more gas. Any ideas?

Oh and when I say knocking, I don't actually mean knocking, just a knocking type of noise.

 

May be way off, but when my 5.4 started breaking exhaust studs(5.4 v8 and 6.8 v10 are famous for this),what you described is just about how it was acting and sounding before exhaust got worse and i fixxed it.you might want to check exhaust manifolds and studs if everything else checks out. These triton engines usually will not knock and if they do it wont be long before they let go.

 

Sounds like a piston from your description, if it goes away on throttle and not hearing at idle. Assume the oils been changed and you have checked the level?