Unfortunately my wife's phone had hissing/crackling coming from the back of the phone. We have an Apple Store about half hour away so we went into there to get a replacement. Her phone is the iPhone 7+ 32GB Rose Gold. We went in and the Apple Store tech support check in guy pretended like he had never heard of this. Maybe he hadn't, I don't know. So we went and sat down with a helper guy and he also had "never heard of this before". He also seemed quite fake. I want you to know, I have owned 3 different iPad's and 6, 6+, 6S+, and now 7+ and only the 7+ has this noise.
Anyways, the Apple tech guy went to the back room and had to call Apple tech support because he didn't know what to do. Apparently its an "emerging issue" he said but Apple has no instructions on what to do with it yet. So he just replaced her phone. Well, the new one, albeit slightly quieter, has the same issue!
Also while in the Apple Store waiting for the guy when he was in the backroom I went and listened to a couple iPhones on the display. It wasn't very busy so it was quiet enough and I heard it on a few of the 7's and more of the 7+'s but not all of them. Some were pretty loud too since I was in a store mind you and I couldn't hear my wife's so that pretty loud. I thought hers was loud but man.. In addition, I asked a person next to me, who I saw had a 7+, who was getting his macbook fixed or something, if he had any noises but he said he had a slight sound but not very loud so he didn't think anything of it.
I get my matte black 7+ 128gb on Monday. I guess we will see what that about.