Undertake a course in electronics practice and theory - buy some basic tools and test equipment - is the flippant answer.
You may be hearing a mains-frequency buzz - which suggests a problem in the power supply area of the device - the DC from the power supply is not being properly rectified and smoothed.
That's where I would begin... DC power rail.
An oscilloscope would be a great help.
https://www.youtube.com/watch?v=eVB_n4t57Yc
https://www.youtube.com/watch?v=8VEg6L2QG5o
Good Luck..., August 2016
