Haven't had a iPhone since the 6 Plus, been on Android, moving from a 6P to the 7 Plus. I got a Week 40 model , Verizon 128 GB 7 Plus and hear absolutely no hissing sound at all from the back of the device/all over while recording a 4K video in a very quiet room/area. I am quite familiar w/the sound of coil whine,etc in electronics, but I don't hear nada from my 7 Plus's body with my ear hunting all over the phone for some sound in a very quiet room. I did have to return my 1st 7 Plus due to dead pixel/dust under the display and I didn't test that one but this current one I have makes no sounds.
I tested my 6P with the 7 Plus in very quiet condition(both recording in 4K) and they both give off actually a similar white noise audio in very quiet conditions but the 6P actually had some weird like line interference actually coming in which I actually never heard until I tested vs the 7 Plus, the 7 Plus sounded better when it is quiet(The mic pickup).
I do feel like there is a little more more like white noise pickup from the audio though then usual in videos when you record, not drastic but slight..weird because I don't hear the actual hissing at all though from the phone,maybe it is just me in regards to hearing more "background white noise" in videos when recording. But no hissing at all coming from the device.