Being any distance from Elstree didn't seem to make any difference - it happens when I am a couple of miles away and happens when I am 100 miles away. I'll try switching off other bits of equipment tomorrow. As I say, it's only when the audio panel has box 2 set to 'phones'. This suggests that the background noise is coming FROM box 2, but it only comes when you key the mic and are tuned to certain frequencies on box 1. I will also try different frequencies on box 1 to see if it still does it, or if it is some magic associated with 122.40.
Next time someone with similar kit is able it would be great if they could try it and see if it happens to them as well.