I'm sure a thousand people have brought up the subject that every ohv 4.0 owner eventually faces - valvetrain rattle, thinking theirs is unique. I'm not beating them over the head mind you... I'm no stranger to it, but this 'new' problem (maybe problem(s) ) is stumping me. Facts are as follows: engine has 112.6K documented miles previous owner had WAY too heavy of oil in it (20w50 AND lucas additive) it had badly worn rockers and pushrods when I got it (sounded very much diesel-like and horrible slop in the valvetrain components) oil pump IS good (+/- 30 psi warm idle) I ordered brand new hardened pushrods (melling I think, memory fails me sometimes) but I did confirm they're heat treated. I ordered the "better" rockers on rails with the hardened pushrod seats (term may be incorrect of course) I have changed both the filter, oil (twice oil, 3x filter) in 1k miles, found no metallic particulates even after inspecting the filter directly, and even the filter mount to a remote mount kit although I find that last bit irrelevant. It is now running the recommended 5w30 oil, with yet another fresh filter as of 2 hours before typing out this post. The rattle is still there, though not as bad as it once was. It is verified to be upper end rattle, nothing in the crankcase. It does not necessarily get louder/deeper/etc when warm, or cold. Oil is not contaminated that I can tell, with inspection of the oil drained, with limited to no sludge ever coming out of the pan. My problem is this: While the rattle is nowhere near as bad as it was, it's still very much there as I said, and the symptoms seem to have reversed in how to reproduce them. Instead of the rattle increasing under load, there is NO rattle under load unless between 2k rpm and 3k. In fact, there is no rattle PERIOD above or below that number, except at a warm idle where it's consistent... but only on the back passenger side. It is also, when throttle is applied under no load condition, intermittent (although once in a while it stops) within that 2k-3k range. I have no doubt that there's something I've been missing here. The only components that I feel could cause this which I have NOT replaced are the lifters and the oil pump. I saw no need to change the pump with the pressures being so good. I don't see how the lifters, being rollers, could go bad with so few miles on them unless somewhat plugged up (though it does run VERY smooth) unless running that heavy oil did more damage than good. But then begs the question, why put the heavy oil in in the first place? This truck is only a part time driver, so while I don't much care so long as it runs, I know it can and should be better than this. My 91 Explorer has almost if not 3x the miles on the clock and DOES run better than this is right now, with nowhere near the rattle/knock/ this one has, and has never had the valve covers off let alone any valvetrain work done. Any help, advice or even reference to some sticky/thread here would be great, and I'd be happy to answer any relevant questions about this to help me diagnose as I am able. Thanks!