I am starting to get more of this engine knock lately, I'm wondering if I should just do engine flush? When I start the 350 up it runs fine and no knock whatsoever up to 5500 rpm when COLD ... However after engine temp meter reaches about 90º and I am trying to floor it past 3500 rpm it starts knocking, after 4200 rpm IT IS KNOCKING LOUD. wtf? Is it carbon build up inside the eng that when hot causes fuel to explode prematurely (it should burn smoothly not explode) and knock and it happens only when it's hot. Help please.

First, it is not advisable to operate a cold engine at higher RPM (higher loads on crank, bearings, and reciprocating parts) since the pistons and rings are not expanded to full size, oil is not warm and flowing, and crank journals and bearings are not at full size. Oil clearances can be excessive, allowing hammering and intimate metal contact. Ring gaps can be wide, allowing combustion gasses to leak past. Pistons can be loose, allowing them to tip and wear skirts excessively.

That aside, detonation is often caused by carbon deposits that both crowd the chamber and become incandescent and ignite the fuel prematurely. Cleaning the chambers with water injection is among the most effective means, and of course that needs to be performed with a fully warmed engine. Commecrcial engine flush treatments are more expensive, and maybe almost as effective as water, providing they contain a lot of water (like SeaFoam). Water, however, is more effective and a lot more plentiful, not to mention more economical.

Another contributor to detonation is heat. Combustion chambers that are not cooled effectively can create detonation. That implies that both the cooling system and EGR are intact and effective. If you haven't serviced the cooling system regularly, it might be a good time.

Lean mixture can also contribute to detonation. Diagnosing that can be a completely separate topic, but having the ECM maintian a correct mixture will require an effective O² sensor.

Doing as advised!