It could have been recorded in the aircraft (running under its own power or on shore power), or in the shop running off a bench AC power supply. Either way, if

It could have been recorded in the aircraft (running under its own power or on shore power), or in the shop running off a bench AC power supply. Either way, if the unit's power supply filter capacitors are degraded (which is very likely), then the AC power supply frequency will leak into the audio output. I deal with this nonstop servicing audio amplifiers from the same era as the the B-58.