My iPhone 7 Plus has the same issue. I can't hear the Hissing that much by playing videos or games, but if I'm recording a video and play it back I can hear the hissing noice clearly. you can hear it even better with headphones on.
To get an even clearer picture, I made the same video with an iPhone 6s Plus and send both same videos to my iPad Pro via AirDrop and listened to them with the same Headphones.
The video from the iPhone 6s Plus was completely silent. The video recorded with my new iPhone 7 Plus had a constant high pitched noise in it. It isn't changing much, just this constant cheeping noise.
Sometimes you can even hear the Cellular Noise kicking in. Never heard that on my 6s Plus.