Knocking on STK392-110 no result, but knocking on the metal can box
for convergence generator makes little noise lines in the picture,
also bit different results if I knock around on the signal mainboard.

Is there any known issues with 51SWX20B convergence problems?

While ago we had to replace that convergence generator box on a older
Hitachi. This convergence generator box is located under the CRTs in
middle on the "floor" of cabinet for very intermittent extreme pulling
apart at tune of 400 for this tiny box, replacing caps did not repair
this.

Cheers, Wizard