Ok after having my first device replaced due to overheating while on the charger, I'll just tell you this. It's going to "hiss" when you first set it up. It may "hiss" all day long. This is called coil whine and it happens on every processor. The iPhone 7 has a quad-core processor that uses a lot more energy which is why you hear it.
The Law of Conservation of Energy states that energy cannot be created or destroyed, just transformed from one form to another. These forms can include kinetic and potential energy as well as light, heat and sound. Energy can also be transferred between objects. Since energy cannot be created or destroyed, the amount of energy present in the universe is always the same. It is simply being transformed and transferred over and over again.
I would be concerned if the device get's extremely hot or makes a noise after the first day and a half of use and the phone is idle or performing a small task such as reading email, checking your bank, etc...
The reason I replaced the old phone is because it got too hot while charging, hotter than the iPhone 6S on the charger, and it was making an audible noise after 2 days while idle. My new phone stopped making noise after 12 hours had passed from the initial setup.
