sorry to hear about your motor issues... I hope it's something simple... spitballen-just a thought, do you have more than 3 full threads sticking out the bottom side of the rockers on the adjusters ??, if you do, you have either adjusted the rockers too tight or wrong pre-load or too short of push-rods more likely... you will get an annoying clicking noise & oil starvation at the pushrod tips/cup/adjuster balls, causing noise & eventually failure of the adjuster tips/balls, very common overlooked issues, with adjustable rockers & over the counter not custom length push-rods... Good luck
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exhaust looks great, Tig/Heliarch welders are the bomb {great} for sure
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possibly the push-rods hitting the heads on the side of the intake ports, especially if you have larger 3/8" push-rods & 1.6:1 rockers...
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if you have stock HP manifolds, not headers, it could be the thermal choke flapper rattling on the passenger side too, especially if the thermal wound spring is weak...
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the noise could be also if you are using shafts that only have oiling holes on one side of the rocker shafts, that then need to be oriented, were the shaft is with the holes down & pointing toward the outside of the engine in orientation, if you have some types of aftermarket rocker shafts they sometimes have oiling holes on both sides, so it doesn't matter... just trying to think of what else could be causing the noise, besides bad lifters possibly