Strange knock activity around 6 000 RPM

Hi All!

I have a problem with my 4GR-FSE.

It's a 2.5 , NA, DI enigine with dual-vvti, 6-speed auto.

WOT AFR is at 12.5

100 RON pump fuel.

It's core is stock, but I run PPE long-tube headers and full stainless exhaust.

Some intake modifications, but nothing crazy.

It's stock HP is 210 crank, now after I tuned it and hardware changes, expect around 230+ crank HP.

Sometimes it detects knock at around 6 000 RPM. Especially during long-pulls like 3-4 gear 140 KMPH +

Spark is around 20 degrees in that range and I keep it flat from 5-6 RPM, so it goes ok until reaches 6k, then the engine starts detecting the knock up until around 6500, but by then it's already retarded a degree or two.

It would start retarding the KCLV (toyota long-term spark correction), based on the KFV (knock feedback value) it sees.

Interestingly, it could run perfectly fine on the same spark advance when it's cooler outside or maybe coincidentally something changes but then start retarding it again on the next pull / day. Interestingly, it would keep retarding after already retarded the spark, so I doubt it is SO knock sensitive in that area, especially when you are already far behind stock ignition advance in that area.

That's driving me nuts. I expect either carbon build-up causing spontaneous heat-spots or something's sometimes rattling against something... Replaced most of the peripheral hardware (mounts, sparks, coils, pulles, water pump, removed AC) - still the same...

Could that be the headers themselves rattle against each other as they expand?

The engine is not perfect, but well-maintained.

I tried to reduce overlap in that area essentially to 0, retard the spark in that area (hard to say if it helped, because by then it's far lower than stock already), tried different leaner and richer AFR.