Are both units positively grounded / earthed? From the same plug if possible.
Is the USB lead removable from the printer? Obviously checking that the metal of the plugs on the lead has good continuity. ie the braid is solid to the plug cases. If not...
Time for some substitution.
If it is indeed a sound card, firstly is the back plate screw solid to chassis? Or, could you borrow another one to try.
Will the dealer let you try another printer?