[linux-audio-user] hum from laptop soundcard

Hi, I have a new Dell laptop with a headphone jack as the only external audio output. With headphones, the sound quality is good and there is little noise to be heard. However, if I plug in my monitors, the output crackles and hums, regardless of the mixer levels. There is 217 hz constant hum, a 125 hz hum when moving windows, and a 689 hz whine when I run Pd. The output crackles when I move the mouse (external USB). I just noticed that it goes away when I unplug the AC adaptor. I think I remember a similar problem coming up on this list before, but I had no luck searching the archives. How can I solve this? Thanks, Ben