As my iPhone 7 Plus was restoring from iCloud, I thought I noticed some sound. After picking the device up from my desk, it was clear the sounds are coming from back of the phone, possibly from the CPU. It seems to get worse if the iPhone is under load, and can be heard while the phone is sitting on a table. Updates can be found here: https://512pixels.net/2016/09/iphone-7-plus-logic-board-noise/