So, now I have an update: The speaker is fixed now! Yay!

Goldenear sent me a new driver, which I put in this Saturday. In the end it seemed to take away the rattling. But there was still that squealing sound, which seemed more noticeable now the rattling had gone away. In the end I fixed that too, but it was a crazy journey:

I talked to Sandy on the phone Saturday and told him that both the rattling and the squealing went away if I took out one of the drivers on the front, and he explained that the passive radiators work by having the air pressure inside the cabinet move the radiators (which I didn't know), so if I took out a driver there would be no pressure inside the cabinet.
As he suspected there might be a problem with one of the passive radators he asked me to take out the radiators and examine them, which I did the next day. They all looked perfect and identical, so I put them back in.

I then thought about what he had said about the air pressure inside the cabinet, and I thought that maybe air was coming out somewhere. I then put the grill and the foot back on, as I had removed that when I tightened all the screws on the speaker, as I thought maybe air was coming out of the screw holes.
But when I put this back on, the squealing sound was still there. Earlier it seemed to come from the front of the speaker along with the rattling, whereas now I couldn't really place where it came from, but it sounded like it came from the back of the speaker.

And then I realized what it was:
When I heard the rattling the first time and then pulled down the sock to tighten all the screws I found that one screw was missing from one of the passive radiators, so I took one screw from the back where the amplifier is attached, as I figured that it was better that the amplifier missed one than one of the passive radiators did.
So when I put my finger on top of the empty screw hole by the amplifier, the squealing sound went away. If I took a screw out by the radiator instead, the squealing sound came out there instead.
Originally, the squealing sound had come out of the front of the speaker, as air had come out of the screw holes for the grill.
So I put a screw in place on the amp, reassembled the speaker completely and could finally stop obsessing about all this. Yay!

Like I also told Sandy when I called him yesterday to tell him about all this, if anyone else ever hears such a squealing sound when playing deep bass, the first thing to check is that every single screw in the speaker is in place.

By the way: If you like, try to listen to the first song from "Blade Runner 2049" (the song I used in my sound clip and also for all this testing) to hear how much deep bass there really is in this song, creating all this air pressure:

Nice detective work. Glad you were able to sort it out.

As you could perhaps tell, I was being driven a bit crazy in the end, so I'm glad that I figured it out in the end .