Tråd: Trappa som knarrar

This highlighted in red is the 'vagnstycke'.

You guys rock! DIY passion vs language barrier:) Thank you. As I understand, the recommendation is to use this triangle profile I am attaching in the picture, between the plansteg and the long wood piece along the stairs. This I will do on the left side, if the problem does not get solved by tightening the screws on the right side. I started to hope again.

Correct, thanks. And my understanding now is that I should use something similar to what I highlighted in Yellow but applied in an area like I highlight here in Blue. But maybe below the step and not above.

The red one is "plansteg" and the blue one "sättsteg" (which you don't have).