Even when it's powered off? Never heard of that one before.
1) try balanced cables (TRS cables) rather than unbalanced (TS cables). Also, if you have a digital input on your mixer try the digital output of the PC3 to the digital in of the mixer. Either one of these solutions should eliminate noise. I do notice that KB3 programs exhibit a little bit of noise, almost like it is emulating a slight B3 motor hum, but it is faint. Does it happen everywhere or only at your home? Can you take the keyboard outside the home and plug it in and see if you get the same results? It could be the electric lines in your home. Can you "record" it and post the MP3 or wav file so that we can get a better understanding of the noise?