I have the same issue too: Just set up my new MacBook Air tonight (Dec 2017) and noticed immediately. Thought I was going crazy- but tested on many platforms and recognized it each time (and even made my roommate take a listen- she heard it too).
Called applie care right away (within my time to return it so I want to be on top of my game).
Specialist brought me through a few different steps:
Checking system preferences- sound
Restarting computer and pressing numerous keys to restart things
Updating software to be most up to date.
Nothing helped. Still hearing it.
Also hearing static from the left speaker. You can hear it more so when it's just voice (like Siri)- but sometimes in certain songs as well.
I'm surprised I'm getting this issue in 2017 seeing this thread has been going on for a while...
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