Well-known Youtuber and Headfier lachlanlikesathing discovers that the iPhone 6s and the 6s Plus suffers from bad design (likely from the additional of the taptic engine and the new pressure-sensing display unit) where the isolation of the headphone port from electrical noise is less than ideal – sensitive IEMs and headphones pick up audible buzzing during normal operation of the phones.
Below are the videos he has posted regarding the issue, and here is the thread on Head-fi discussing the issue.