That sounds like a possible static or grounding issue - please check with the DDJ-SX3 connected to a different electrical circuit than the one where you noticed that issue, and connect only the computer via USB (if it's a laptop, please try without the power to the computer). See if the problem persists in that configuration - if so, then it may be that the SX3 requires servicing.
Have you got a black dot beside the serial number?