Rattling sound under low RPM acceleration

[Buurin]

For the past month I have been hearing rattling sound under low RPM acceleration. Like under 2000-rpm low. It isn't as loud when idling.

I read the plugs today and see carbon buildups on all 4 plugs and pistons. Some carbon and oil apparently made it into the spark plug threads. My guess is carbon deposits raising compression enough for the engine to knock on regular gas and it's time for seafoam or steam. Yes I searched and found piloter post about gradually running a gallon of distilled water through PCV. I have no less than 5 jugs of 4L distilled water, meant for radiator flush. Might as well put them to some other good use. Other possibilities would be aging serpentine belt tensioner; could also be timing chain tensioner .

Question is, do I have all bases covered?

[Bitter]

does it do it in neutral as well?

[Crowbar911]

Mine does this under a light load, and right before shifting into 2nd. Not sure whats up with it, but its been doing it for 30Kish miles now. I cant figure out what it is.