Do you have the FM9 plugged into a properly grounded outlet?
If the noise level doesn't change when you turn the front panel OUT knobs, then it's likely something post D/A converter in the analog output section.
I'd wager if it was external interference, the more sensitive input section would be picking it up more and running through the preset. Do you hear any normal guitar signal or effects through the unit along with the noise? If so, how loud is the noise relative to the guitar signal?
Has it always made that noise, or did it work normally and then just start doing that? I don't think it will make a difference, but you could always try and reset the system parameters from the Setup > Utilities menu in case it's some kind of weird issue there.