http://www.digitaljournal.com/tech-and-science/technology/some-iphone-7-handsets-are-making-strange-hissing-sounds/article/475219

iPhone 7 owners complain about annoying 'hissing noise'

Posted Sep 19, 2016 by James Walker
The iPhone 7 has only been available for a few days but an issue's already been discovered. Growing numbers of users are reporting an audible "hissing noise" can be heard inside the case. The problem's believed to be caused by poorly-damped coil noise.
The iPhone 7  which comes without a headphone jack  sent the company's shares falling late in ...
The iPhone 7, which comes without a headphone jack, sent the company's shares falling late in the week to a still market-topping value of $572 billion
Josh Edelson, AFP/File
The iPhone is now known for coming with a minor launch issue. Apple tends to work the problems out with time, leaving early adopters to document the annoyances. This year, "Hissgate" is making the rounds, a loud hissing noise that can be heard emanating from the iPhone 7's back cover when the processor's under load. According to some users, it can be heard from a considerable distance away.
As Ars Technica reports, one owner of a new iPhone 7 Plus reported hearing the sound while his device was restoring from iCloud. "It was clear the sounds are coming from back of the phone, possibly from the CPU," Stephen Hackett wrote on his blog. He used a microphone to record the hissing noise he heard. Hackett claims it was audible when the iPhone was "just sitting on the table," although he has not disclosed how close he positioned the microphone to the device.
Apple has not officially acknowledged the problem. However, people who returned their phones to Apple stores have been able to obtain a replacement device. MacRumors collected a set of posts from its forum users that suggest the company's support staff aren't yet aware of a widespread issue. However, the sound can be easily reproduced by watching a video or starting the device, convincing Apple to offer replacement phones.
The hiss is currently being blamed on coil noise, a common occurrence in electrical devices. Coil noise, also known as coil whine, occurs when components resonate at just the right frequency to cause physical vibrations, creating a noise. Increasing the current in the circuit strengthens the resonance, creating a louder noise. When a device is placed under heavy load, more current may flow to its components, creating the iPhone 7's hiss.
To help avoid audible sounds being created, devices contain some form of damping to prevent components vibrating together. It’s possible the iPhone 7 doesn’t have enough damping to mask the coil whine, allowing it to make the soft whistling, hissing and crackling sounds that users are experiencing.
The current consensus is that "hissgate" is nothing to worry about at present. You should be able to keep using your phone without worrying about negative consequences. Similar sounds have been heard on other devices, including older iPhones. This year's handset appears to be particularly audible though, drawing mass attention to coil noise.
If you become annoyed by an especially loud device, you should be able to have it replaced at an Apple Store. Apple Care staff are likely to become more familiar with the problem over the next few days, judging by the large number of reports on social media. Actually receiving a new phone could be problematical though. iPhone 7 and 7 Plus handsets are currently in very short supply and Apple is struggling to replenish its stock.