I've had a similar questionable sound / problem last year. Total lack of power and bad crack / tap sound under hard acceleration. Same thing... let the engine cool down and started fine, then after a few minutes it was back and worse. Seemed to be worse when driving hard.
My issue was exactly (kinda) what Vegas69 said.
The lifters were starving for oil due to a low oil level.
When the oil was cold and thick there was no problems. Once the oil temp increased and the oil was thinner (plus the low level to begin with) the lifters were starving under acceleration because the oil pump couldnt suck any oil and what it did suck was as thin as water.
We did a fresh oil change and ran a heavier weight oil to help save the lifters and it worked. The problem hasnt come back and luckily the lifters werent badly damaged from being hammered. The fact that they were hydraulic helped too. Had they been solid lifters, they'd be toast.
I hope for you that its that simple.
Good Luck and let us know.
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