Turns out there are several iPhone 7 and iPhone 7 units that are making a weird hissing noise when under heavy load. It looks like the issue occurs only when there’s a lot of CPU/GPU activity from the A10 Fusion. There’s no official confirmation of the same, or why only a few units are seemingly affected.
The iPhone 7 does not have any fans or liquid-cooled internals, so there’s no clear explanation as to what’s causing the hissing sound. What we do know is that it is definitely not a feature. It is likely that a few units from the initial production run are defective, leading to the hissing sounds emanating from the phone.
The iPhone 6 suffered from “bendgate”, wherein several units would easily bend under not a whole lot of stress. To fix the issue, Apple went with a higher grade of aluminium in the iPhone 6s. The hissing noise isn’t unique to the iPhone 7, but it is noticeable now because of the sheer number of launch-day units being sold across the globe. The phenomenon itself looks like a form of coil whine:
If you’re also hearing a hissing sound from your iPhone 7, your best course of action is to take it to your nearest Apple store and get the phone swithced out for another unit. Given that the iPhone 7 is in a lot of demand and there’s not enough stock to go around, best get to it fast.