Crackling From Speakers

I have a DJ mixing package, which i use 2 soundcards. The 1st is from the nVidia G-Force mother board and the 2nd is Creative Labs Soundblaster. The problem i have is when i have the software running and i tweak any of the dials, volumes or crossfader a really bad crackling sound comes from the speakers. This is also heard in the headphones when cueing in the 2nd track. My PC is new and cost a lot of money.
I have also tried it no my PC at work and it does the same thing. Surely i’m not the only one to have this problem and what is causing it.
Please, Please can anyone help? – Regards STEF