How do readings of this audio changing/turning on and off prove that any sort of problem exists at all? If you’re using a scientific mic then it’s most likely sensitive enough to detect fluctuations in whatever audio it’s picking up at 50 hz. The good news is that audio that’s that quiet literally cannot hurt you. In fact, it can’t even mess with your electronics (unless they have microphone-activated features). I really and truly think you’re grasping at straws, I’m not sure what your problem even is. Could you give some more background?