It's very possible that what we are seeing is due to yield rates on the chips. Those that are well into the "good" range don't hiss. Those that are "bad" get rejected. It's possible that chips close to the "fail" or "bad" cutoff need to be "fail" to allow Apple to ship phones with no hissing. That would, of course, slow down production rates and delivery dates, fwiw.
[doublepost=1474201734][/doublepost]OK, this is vaguely interesting. I have two iP7s here right now. I downloaded the Geekbench CPU benchmark append ran it on both. One has a very clear, loud hiss/click noise under load. The other is basically silent - I could occasionally hear faint clicking if I was in a VERY quiet room. I can't hear any noise from that phone in a normally noisy room.
I do think it's normal manufacturing variations and chip yield rates that are the cause.
Being that you have two iP7s and one makes this more pronounced to me makes this issue warrant a return. I get it that they wont be completely silent especially putting your ear right up to the back but to hear it from a distance is concerning. If your assumption of chips is correct, I do not want one on the lower end of the spectrum which may essentially fail quicker in the long run.