Awful knock at high RPM

Good morning,

Can you upload a video of the noise so I can hear it?

It is either a rod bearing or main bearing from your description.

https://www.2carpros.com/articles/engine-noises

A flex plate would sound like a valve tap.

Roy

Main bearings don't usually knock but rod bearings do. Drop the oil pan and plasti-gage all the rod bearings.

That is when the clearance is checked between the bearing and the crankshaft.

Roy

You are welcome.

We are always glad to help.

Roy

To use plasti-gage, clean the motor oil off a rod bearing in the connecting rod end cap, lay across the bearing 1 string of the plasti-gage, then install the bearing and cap, torque it down to spec, then remove the bearing cap and measure how much the plasti-gage string got smashed and that tells you the amount of oil clearance and if it's too much, it means the bearing is worn.
Here is another source for how to use it: https://www.plastigaugeusa.com/how.html