legacycentral bbs

I know there's another fresh thread about the same device, but my problem looks different.
CEL, 22, 2 other knock sensor - used but checked - plugged, still the same. KS rewired - no change. Harness checked. CEL show up usually when engine is hot and almost always above 3000 rpm. Sometimes it goes away after few seconds, sometimes it stays. Of course no WOT from my side.

Just one quick question - afaik CEL doesn't show up when there is a knock - it's only primitive resistance checking, right? So there is no way that ECU sees some knocking, tries to change ignition, then still sees some knocking and finally 'realizes' it can't do anything about it so it throws up CEL?
There's a suspicion some internal part of the engine may cause noise similar to knocking. This is what my mech is saying but I got some doubts about it.

KS from imprezza unfortunately. No way to find original.
And another important notice - KS plugged but hanging freely - not mounted to the block - rough engine work but no CEL.

There are some differences between the older Legacy knock sensor and some of the newer Impreza knock sensors. Whether that really is the problem, I'm not sure.

The engine really shouldn't run any worse with the knock sensor not mounted to the block.

Is this all on the same car that you just replaced the ECU?

Yes it is.
We replaced once again KS for the third one. 10 miles today and no CEL. Still imprezza stuff.