Early buyers report, that their new iPhone 7 and iPhone 7 Plus are making weird hissing sounds when the phone is under heavy load. The sound apparently originates from the back of the phone but not every iPhone 7 seems to suffer from that problem.
http://www.notebookcheck.net/Apple-iPhone-7-Does-yours-also-make-hissing-sounds-under-heavy-load.174939.0.html
MLCC acoustic resonance, two ways to solve
(1) PWM IC changes the operating frequency (only alleviate the situation)
(2) MLCC capacitors can be replaced by solid-capacitor (100% resolved)