Does the laptop power cable run adjacent to the monitor cable?
I'm guessing it's interference, either from the cables running parrallel, or from badly shielded connections in the laptop.
If you're lucky then a bit of adjusting the cables, or something metal to seperate them will fix your problem. If you're unlucky it's internal shielding on the laptop. (If you're not that unlucky it could be a badly shielded connector on either laptop or monitor