I own a 2010 Mazda 6 2.0 MZR-DISI
And I absolutely love the car to bits. There is however a problem that I find very hard to troubleshoot

It makes a rattling sound when hitting higher RPMs (around the 2000 mark) while not, or softly accelerating (especially when going down hill with cruise control) , the rattling is almost non existent when cold, but becomes more and more audible when the engine heats up.
Initially when I heard the slightest of rattles, and having gotten a timing error code, I replaced the timing chain (and guides and springs etc) of the vehicle with a mechanic, after which the problem seemed to only get worse.
But seeing the fact back then the performance wasnt suffering, it seemed like a minor annoyance which due to having to immigrate was a problem for later.

Now the rattling has slowly become louder, still not as much when cold, and increasing when the engine heats up.
The car starts to stutter, and lose power when pressed around 2000RPM or accelerating from 60KMPH in 3rd gear and 80KMPH in 4th, and feels almost as if one cylinder is not wanting to play game?
The rattle seems to come from the top of the engine block. and I have no idea what could be the issue. as I cant find a similar case anywhere.
And having a mechanic play detective for a week would be more expensive that the car is worse, and replacing the engine would also be about the entire value of the car?

So please wonderful people of this forum,
could you think along with me, to what it might be?
is it due to a timing issue? or are there thing I could check to narrow down the problem?

Yours sincerely
A desperate Mazda 6 owner