After some further listening, I think I was wrong and I suspect a blown speaker instead, after an accidental extreme high volume blast about a week ago.

I have crackling/static and sometimes complete audio loss in the right earcup of my QC25s, especially when touching/moving the earcup, but noise cancelling still works in both earcups so I don’t think this is the known defect mentioned by @bob above. It happens regardless of connection method.

I suspect a faulty connection to the speaker or board in that earcup so I’m going to open it up and see if I can find out what’s damaged. Just wondered why there isn’t a guide for fixing internal connections – or are they implicit one or more of the other guides?

In my Late 2012 27" iMac I Scotch-taped the SATA connector to the case to stop it from pushing against the screen after reassembly.