try puttingan isolating transformer or DI box in your analouge signal chain (ie, between your soundcard and your monitors).
Laptop PSUs are a real pain like that. Because of the switching nature of their design (and the fact that they weren't designed with audio in mind).
I run into this problem all the time. I'm not entirley sure, however I think it's a ground loop which is being switched by the PSU (Ie speaker earth->montior->laptop->psu->laptop earth), hence the difference from a "standard" ground loop
bad name- average site...
I spend my days getting paid to push around air.
That fact is both amazing and depressing.