John- re the noise problem.
Were the poles with the 'noisy' down wires actually phone line poles or
power line poles? Analysis you have done so far suggests the noise is mains
related and sourced so it would be surprising if the poles in these cases
were phone line poles; but possible if the noise was coming from the
exchange, a possibility you seem to have eliminated at this stage. Have you
listened near a power distribution xfmr yet? Could be either pole or ground
cubicle mounted depending on area etc. Perhaps the noisy down wires were
pole mounted transformer grounds? Noise on the earth wires from distribution
xfmrs  may help indicate if the noise is on the 11 or 33 kV network feeding
it/them or not. If you can establish which side of the xfmr has the worst
noise you may be on the way to localising the source but a clipon CT on the
ground wire/wires may be needed to prove this as receiver listening can be
counter-productive due to the radiation from all the myriad wires the energy
can couple into and propagate quite long distances over.
73
Dave
ZL3FJ