hey guys, im sure there are probably a few threads on this alreay but i cant seem to find any

please point me in the direction of one if you know though!
alright, been djing for ages, and know enough bout production/computers etc. anyway, only just upgraded my setup and wanted to crank some give away CD's!
Alas! Recording seems to not want to work.
Iv got numark TTs, Pio 800s and a DDM4000 (has S/Pdif output but at the moment are only using normal RCA out)
I have ran 2x RCAS from the "booth/record" outputs into my line in on my desktop computer (pretty decent comp), but when i record (in any program) i get a lot of hum/buzzz underneath the track. Its not the track or the input because it does it even if the track is not playing/mixer is even off!
Could the line in plug behind my computer be picking up interference from power cords etc and making the buzz?
Was thinking about getting a new soundcard with S/PDIF in/out and using that but if thats still not going to work then im not going to waste my money.
Please if anyone could help that would be great.
OR any other (easier) way to record my mixes!
cheers guys
Scotty V