problem with cd decks (numark axis 9)

I just recently bought meself a pair of brand new numark axis 9 decks. They are very, very nice but there is a slight problem with one of them.
When I'm playing it, there is a constant sound of popping/clipping. The ironic thing is that it only ever rally happens when the tune is ambient or during a breakdown. Not when the music is pumping. The other deck appears to be fine and it happens regardless of whether it goes through the mixer or wired direct to the amp.

Any ideas???