This is the answer & it's in the faq. (and it's the link given by phadedvision)
Why do I hear a high pitch or rumble sound ?
Experiencing a continuous rumble sound? There is a simple solution!
If you ear a muffled rumbling sound, this is surely a ground problem. This happens with laptops that have a power supply with a ground connection (3 pin plugs). The problem will stop if you unplug your laptop. Since you can't always DJ on battery mode, you need to get rid of the ground connection by using an adapter of this sort :
In fact it's a normal plug without ground. Some computers with AC do a "ground loop" that's strange but it's your case
