creaky sound on access virus C

[maco.maco]

Hello,

on some pacthes,my access virus c desktop,produce a creaky sound,like as guitar with dirty pots.
this strange noise happer indipendently from the velocity/sensity of the played keyboard.also if I move the volume pot,the noise follow the main outpout level...
I hope that there'no hardware problems on my synth... :(
anyone had the same problem?
thanks!
Marco