Is the problem Win 7/64 machine a desktop?
Does the amplitude of the recorded noise vary with the gain control on the FastTrack II or is it constant?
Do you get the noise if you use a mic rather than the instrument input of the FT II?
Do hear the noise on playback of "clean" files recorded e.g. on the HP notebook?

yes, it is a desktop. I have to say I haven't checked the noise in recordings so much (with so many variables), but I can tell you that a couple of days ago I was recording with the synth and the noise DID get in the recording. as regards your question: the noise DOES vary according to the gain/volume control, which means that if I don't turn it up that much, I can barely hear it, but I get to 30% (approx) and there it is, constant. the more I turn the volume up, the higher it goes.

I haven't tried the mic simply because I have it somewhere else (not at home!), but my guess is that every time I plug something (whatever) into the inputs, the noise is there. if I take everything away, the noise is totally gone.

I've just realised that the noise is NOT THERE if I play back a song, say, from the Windows media player that was previously recorded (or from any band)

any ideas on this? would it help if I used TRS (balanced) cables from the instrument to the fast track?

update: I've just tried TRS cables from the Kronos synth to the fast track, NOTHING happened, the noise is still there. BUT I have to say that I think I've narrowed it down to an ELECTRICAL problem, since I've noticed that if the synth is UNplugged there's no noise!!! it happens every time I hit the "listen channel" (or sth like that) button in the track controls in Sonar and the synth is plugged to the wall socket. If I unplug it while holding that button on, the noise disappears!!

and I've also noticed that if I choose the fast track as the default audio player in windows and playback any file with the media player, NOTHING happens, no noise! so , there's sth definitely going on with the electricity. is that a ground loop?

OK, I think you have diagnosed the area of trouble yourself: hum in the Kronos output or caused by a ground-loop between the Kronos and the interface/computer system. If it's the second problem, the solution is relatively easy - use a transformer-coupled DI box that isolates the grounds between input and output.

If the hum is in the grounding of the Kronos itself, you may have more of a problem. I don't know the Kronos powering arrangements; does it have an external power supply unit? If so, it may be possible to try a different unit that carries ground through it from the outlet socket to the audio ground out. Make sure the voltage and current ratings are the same on any replacement you try. I'm assuming you have tried plugging the Kronos into different power outlets, specifically the same one that the computer is using.

Is there any help for this type of trouble on the Kronos user groups?

I've finally solved it! and it was so easy I still can't believe it! all I had to do was simply put a 3 to 2 prong adapter, and then another but 2 to 3!! so now there's absolutely NO noise and recordings go perfectly well!

I'm over the clouds with this!

thanks to everyone anyway and for the suggestions! I'll stay in touch!

Well done, but I'm not sure I follow exactly what you did. Does this mean that your Kronos is now not earthed except via the audio output cable? If this is the case, what it shows is that an isolating DI box would give you the solution to the hum problem but also allow you to retain the safety earth on the Kronos. I think you should regard the trial with the mains adapters as an experiment or investigation and now strive for a more permanent solution that meets safety standards.