I might add to the confusion or the complexity - but I had the same sound - sort of a recurring high pitch sound, at low volume, It appeared randomly. At it started recently. I did reset SMC and PRAM, disconnected devices etc.
I started to look a my behavior - if I didn´t do anything - did the sound appear? - well - yes....or no...you don´t always do nothing when you think you´re doing nothing...(I´m building up here..)
well I again grabed my mouse - what! the noise came. I slipped my mouse - sound gone.
Recently I changed my mouse to an old school Logictech (m90 I think) - wired mouse - cause my Magic Mouse and my Wireless Logictech did work proberly (still don´t) - this wired mouse caused the noise. It´s gone - and the noise is gone.
Have a noise..augh nice day