Hey man, thanks for the reply.
It sounds something like 1kHz I'd say.
As for the ground loop, it's almost impossible... My audio interface's chassis is grounded to my computer's chassis, I tried balanced cables and unbalanced cables between the audio interface soundcard and the speakers. I even tried powering the monitors with the ground pong cut and still there. As soon as I unplug the firewire cable, the monitors go back at their normal state. Everything share the same ground point. I might try grounding my audio interface on my desktop chassis or directly at the powerbar's ground instead of going by the computer, but I believe it won't change a thing (not supposed to have a loop there).
As for the ferrite cores on a keyboards, that's a good ideas but I think there are differant ferrite cores for different frequencies. I'm clueless.
I sent 2 emails to Edirol (Roland music.. Boss gears, etc) never had a reply yet. I really don't want to look into another product because that interface and any Roland gear I had so far always worked great. They were supposed to include the snap-on ferrites with the interface but I might have lost them years ago, and it was never a problem with passive monitors...