This good be a wire on wire but not soldered issue somewhere. Sometimes they forget the transformer wire from 50 ohm to 200 ohm ( this is in the transformer) to solder. You can get an electromagnetic field around this “gold on gold” connection.
I got this with an Haufe bv08 transformer and searched 8 ours a day 7 days to change everything in and out with an other mic that I built. Because it was a stereo pair. Nightmare I got from it. Also with a half broken wire or bad soldering broken soldering. It could be it. Good luck with your searching.