I've been reading for a long time, and finally decided to join so I could add to this thread.
I'm on my second Matte Black 7+ 128 GB. On my first device, I did not hear any hissing or anything, but when I placed the phone on a table, and recorded a video for ten seconds, I would hear what sounded like bacon sizzling.
During videos in quiet places, the white noise was present, and seemed to gain whenever there were noises. For example, snapping my fingers.
So I went to the Genius Bar today and they exchanged my phone for me for a new one. The tech could hear the "sizzling", and he said this is NOT normal. So they went ahead and replaced it with a new phone.
I recorded with the phone on a flat surface, and the "sizzle" is still there, but more faint.
The white noise is FAR less noticeable.
First device was serial number F2L, and new phone is F2M. Both from the same factory but different weeks.
So all of the findings were documented. At this point, I think we have to wait for Apple to acknowledge there is a hardware issue, in order to get a "fix". Otherwise it seems to be luck of the draw. Also, there's no telling whether the white noise and sizzling will get worse on this device either.