Just think about it ... the concept of earth bonding ..
All the metalwork, electrical systems and service supplies are communally bonded to the building electrical supply point and may be also be linked to a ground spike. So during any short, spark or other miss fortune the potentials within the buildings earth planes remain uniform, not saying they don't or wont rise , but uniformly .. faraday cage style.
You have established a insulated link to a second buildings earth system , some 175 feet long ..
The average lightning 'strike' has a rise time of 250,000 Amps/micro second, so any parasitic inductance and or series resistance , when subjected to such a 'stimulus' will produce extreme voltages , (reason for flat tape conductors - less inductance)
As the second building will de similarly configured, the possibility exists for a 'pd' to exist between the microphones and there immediate surrounding , generated by purely ground resistance and/or parasitic inductance ...
this may the cause of the noise in the first place ?
Sorry If I'm stating the obvious ..But, 'hay I'm not holding them !
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