It can't be recorded I don't think. I had the same problem on my iPhone 6. Took it in and got a replacement. Only when on charge and grounded. It's a kind of static noise when touched with a finger you can feel it but can't hear until you put your ear against it.
If you pick it up it seems to stop. Very strange. I've had my iPhone 7 128 for a month and just noticed it on this phone.
If you pick it up it seems to stop. Very strange. I've had my iPhone 7 128 for a month and just noticed it on this phone.