Thanks Jim.
It is a 100Hz and 200Hz noise, so for me, it is mains based.
I don’t use a fixed phone anymore, so the base stations have been unplugged a long time ago.
The internet router has been turned off, and it made no difference.
I tried all of this when the family was out, and also turned off the mobile phones.

The noise was still there.
It is intermittent, and low level. My money is it being my air source heat pump, so I need a way of filtering this noise.