loud humming in recordings

have bought 2nd hand minidisc Sony Net MD Walkman MZ-N910.
i am testing it out and seems to have a problem when recording
thru mic-in socket.

i have tried three different mics so i don't think it's the mics.
have tried altering minidisc recorders mic-sensitivity to high
and to low, without success.

all the recordings have a very loud humming in background
which spoils them.

difficult to describe, could say it's like an electrostatic
noise, like if you stand near an electricity pylon
but much louder than that.  it's as loud, or even louder
than the tape hiss i would get if i was recording by
old-fashioned audio tape recorder - and i thought minidisc
was supposed to be better quality.

i can't hear this noise on recordings thru line-in.

is there something wrong with my mic-in socket?
or something else in machine?

thanks