Hello all, sorry to hijack this post, however I do have a bit of info on this topic that maybe somewhat helpful or shed some light.
I own the Numark NS7FX & NS7II, I use Virtual DJ 8 on PC Win 7 Pro. I get this same crackling, popping sound after about two hours of use on both FX & II. I have updated my bios, expanded USB memory, new cables, sent the unit in for repair, used a Mac, another PC, Updated drivers, replaced cables, tried Traktor, VDJ & Serato… NOTHING HAS WORKED! Recently Numark 4Trak users have also reported this problem. The only conclusion left, it HAS to be the Numark Drivers! I never had this problem until 2014 and updated my Numark sound drivers. This topic has really starting to heat up as Numark owner are starting to pour in with complaints about this exact issue on the Numark forums page. If I get a fix or hear of one, I will post it asap.
I have had this problem for over a year and a half now.
If anyone else can find a fix please let us know!
Thanks!