Tubes trouble?

To me this sounds like a mechanical hum. First thing to check would be the screws that hold the circuit boards and transformer are snugged. Then I would call AR tech support and ask if it’s okay to remove one or all of the tubes and then power it on. This way you could remove one tube at a time and see if the hum goes away -- and you know it’s the tubes. (I should mention power off and unplug the unit each time before you remove any tube).

Not a good idea to remove tubes leaving a socket or sockets empty, and then power on. Unless of course ARC directly gives their verbal consent.  Also, even if you could get away with it without damaging the power supply or downstream equipment, it would be difficult to assign blame for the hum, because you've removed the pathway (to the speakers) by which you detected the hum in the first place. So you'd likely get a false positive result.

Something about the way the OP described the "humming" makes me think it is not a 60Hz or 120Hz noise but instead a mix of frequencies, which you can get with a microphonic tube or a tube picking up RF.  Also, those 6922s can do that.

Thanks for all the suggestions so far.
Tried changing the position of the phase and neutral (I live in Sweden where we have 230 V. That is, rotate the plug in the socket. Now it was completely silent, tried to change back and the noise occurred again. Changed again and dead silent again. The problem is fixed .
Many thanks for all the suggestions.
Regards
Niklas

Great. It is always wonderful to have an easy fix!