Mbox/laptop issues

[oedipus_complex]

Hi,
I was wondering if someone can help me troubleshoot a problem with my Mbox (running PT5.3.3. on a laptop). I have the line out plugged into an Alesis RA100 monitor amp, and I'm getting allot of noise (digital sounding). The headphone out doesn't give me any noise at all, however. The thing is, when I unplug my laptop from mains power, the noise goes away. Obviously its completely impractical to just run off batteries however. So I assume the problem is electrical. Other things to mention is that I've tried different cables, different usb cable and port....the noise starts as soon as the computer is fired up i.e. before the OS loads. The thing that really baffles me though, is the problem only just started - I've been using the Mbox in the same setup for about a week with no problems at all.....

[oedipus_complex]

No ideas anyone?