I had my hopes really high when I went to the car, checked the side skirts and realised some of the clips are not fully pressed. But after fixing them, I just started the car and the creaking sound was still there

I really don't understand as the sound can be heard even at really low speeds, 2-3 km/h. I also checked the doors, the headliner, no fix yet.
One strange thing though, yesterday morning I didn't hear the sound for the first time. I drove to a nearby city (aprox 100km away) without hearing it. But later in the day, when I drove back, there it was again. This morning it was still there, but it faded away when it started to rain. Then I realised yesterday morning it was also raining. I'm not sure there is a connection, but I don't know what else to do...