SoftStep 1 hardware uses electroluminescent wire (EL wire) to light the keys — this is the cause of the buzzing sound. The sound should not be present in your audio chain, just physically coming from the SoftStep. The only way to make this completely go away is by turning off the backlighting. This can be done in the Settings section of both the Basic and Advanced editors.
SoftStep 2 hardware should not hum or buzz at all.