Ground loop issues....

[morbid]

as Danny says, If you get no noise with your laptop unplugged and on battery then its your laptop adapter / charger for sure,

You can try using one of the ground loop devices on the outs of your soundcard to clean it up or live dangerously and take the ground wire out of the 13 amp plug on the end of your adapter - usually molded plugs though so need to rewire.

Lifting the ground is a shit solution though and I wouldnt recommend it

Dell sell Euro power leads (no internal ground) which you could use with an adaptor which might work