Casio CZ/ VZ/ FZ - Pro Series

Hi list, I'm the owner of a CZ-1 which I use a LOT.
However, it was unused for 2 months because of other type of work
and now I discovered that the keyboard makes an audible noise on
audio outputs (but not on the headphone) which resembles to zippering.
The noise does not appear when MIDI data is played from the sequencer.
Each every note through the whole keyboard has a moderately loud click
on the NOTE ON and repeatedly at every aftertouch action.
From these I came to the fact that the keyboard is the problem.
I would like to ask the opinion of experienced users and abusers
about how can I fix this ? It must be some kind of global problem
because of the whole keyboard is affected but I don't know where to
start.
As a side note I want to mention that I accidentaly switched the
midi in/out cables and I got a really loud noise (somewhat static but
with artifacts). Is this normal to the CZ-1 ???
The soundcard the CZ-1 was plugged in is an
M-Audio Delta Audiophile 2496 which is a quality card.
Thanks in advance:
        Darth