Since upgrading to OS Sierra on my iMac, I had noticed a horrible buzzing noise coming from my external desktop speakers. The pop would occur after a short timeout and seemed to indicate that sierra had put the audio driver to sleep, leaving no output to the external speakers. The initial switching off of the speakers would cause the pop and then the speakers would buzz until a system sound woke up the audio driver & played a sound.
This hissing & buzzing of the external iMac speakers was driving me mad. A few years ago I remember a fix called Antipop which was a small Daemon which would play a system narration consisting of no actual sound, but enough to keep the sound driver from sleeping.