The issue doesn't appear for everyone. I tested an iPhone 7 using 3DMark "Ice Storm Extreme," a performance benchmark that puts the processor under serious strain, and heard no hissing at all. That suggests that the noise may stem from a manufacturing issue instead of an inherent design quirk. Not that this will make you feel any better if you're affected, of course.
We've asked Apple for its take on the reports and will let you know if it has something to say. It won't be pretty if you run into this issue and want a replacement iPhone, though. Supplies are already extremely tight, so you may end up waiting days to get a blissfully quiet device.