If its been running that long, and you've changed nothing, looks like either electronic/electrical or possibly the Z ways and mechanism might need cleaned / lubed even though it operates smoothly by hand.
I dont understand it changing direction though.
And like Hood said, missing steps/ stalling would make a bit more racket than just a humm..unless maybe you were super micro stepping.
Not too difficult to switch wiring just to check, mark the wires carefully and take notes.
Good luck,
RC