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ZAP.143
Thanks - that covers it - here's my guess & summation, based on a YouTube video (with a lot of fan nose)
OK - If your block is out of a '98, you have a roller camshaft engine - which is good.
What I think I'm hearing in the video is an out-of-specification roller lifter "make itself known" when the rpm's drop from the initial cold start (high rpm) and as the oil warms up, and the oil pressure lowers in concert with the lowering of the rpm's.
Background history to pull it all together.
When these engines were starting to be REBUILT en masse the first go-round between 2005 - 2011, the original GERMAN roller lifters were priced as high as $70 each and a little hard to source. FORD Stealership had them (at a HIGH price) and Ebay had them occasionally BUT they were swooped up as fast as they were advertised.
Although you mentioned OEM parts for some of the replacement parts,
that's NOT mentioned with regards to the roller lifters.
When rebuilding, many guys balked at paying $50 - $70 each X (12) for the Genuine German OEM Roller Lifters, and went the Chinese Aftermarket route at about $16 - $20 each X (12).
Now that you know that, take into account the following; The history of these engines, from the factory, is that they were very tolerance sensitive with regards to the GERMAN factory lifters - and those were manufactured to a very high standard...
Despite the German's attempt at tight quality control, there were LOTS of noisy 4.0 OHV's right from the factory with one or more German lifters ticking. The subject was quite popular on this forum back in the day when I joined - 2005. Overall, the consensus was "just drive it" - it's a truck engine.
But still - many guys gave it a shot and performed a lifter replacement, only to have the noise pop up in another cylinder either immediately or a little time later...
With Chi-Com aftermarket lifters, well, consider how tough it was for the Germans to manufacture them, and then imagine your typical Chi-Com in charge of manufacturing AND quality control of a high precision roller lifter - and you get a clearer picture
That's what I'm guessing is more than likely your problem - a bad lifter - but your mileage can vary.
Perhaps another video with the serpentine belt disconnected is in order
(that way you won't get the fan noise).
Personally, as long as it passes SMOG, I would FIRST give it a shot with some thicker oil (10w30) and see if that helps.
If it does, I'd drive it 'till it drops VERSES tearing it down for one or two lifters.
You could be entering Pandora's Box territory opening up the engine - I've read it here too many times...
HTH -
PS - Your post - #9 - seems to be cut & paste from an e-mail inquiry to the guy that built the engine.
With that said, If you do have a chance to ask about the lifters (again), I wouldn't put words in anybody's mouth - I'd simply ask "where did you source the lifters - they're tough to find"
An answer of anything other than FORD will more than likely be chi-com.
BUT - as I wrote, even the Germans made quite a few bad ones despite their precision machinery AND high level of Quality Control