Strange click through headphones

Hi

I have just purchased a DDJ1000 and Rekordbox software.  All is great except for an annoying click and what sounds like a jump in the sound through the headphones, a sort of millisecond of silence. It is very hard to describe. It is not occurring through the master out and it doesn't seem to cause any issue with the timing, just very annoying like the headphone output can't keep up. Not sure if it something in the software settings  Anybody any ideas?  

Thanks

Mark