Just got a new AudioInterface, something really weird is happening..

my sound card arrived today, got a Lexicon Alpha sound card, (cheap, like 50pounds).
I've connected it by USB to my macbook pro, and and the sound card's RCA line outputs to my krk speaker rca slots, and basically,

I have a second monitor, a DELL 21" screen, which i've connected my macbook pro with mini displayport to VGA, and when i drag say an internet page over to my 2nd display, my speakers start making a loud buzzing noise!?! im using google chrome, ive opened up a couple of tabs, one with youtube and then DJTT community s, theres no buzzing noise when im using my internet/apps on my macbookpro's screen, but when i drag the page over to the dell screen it just starts buzzing, like really buzzing, ive just opened up logic, and was fine on my MBP, but when i dragged it over to the other screen buzzing noise started again, now what the heck is up with this? :-(

Any help would be really welcome as i obviously plan on using my dell monitor, and will most likely always have logic open on there....