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There may be a small amount of electrical noise in the headphone circuit on any product if there is no audio playing and the headphone volume is up. You may also hear interference from other electrical devices on the same circuit, and this can be more pronounced when connected to a computer if the USB circuitry is not properly shielded / isolated.
So long as this noise does not occur through the master outputs, you should be okay in using the unit as audio playing in the headphones typically drowns out the noise. But in the event this can still be heard over the music, you may consider having the unit inspected / serviced - in that case, please contact a Pioneer DJ authorized service centre in your region.
Let us know if you have any other questions.