I had the same problem with my latptop, an ECS Desknote. There was also 
a distinct whirring sound that had a very direct relationship to my 
video card activity, eg. when there was lots of bright things on the 
screen it sounded different than when the screen was mostly dark. I 
figured both problems were the result of poor insulation between some 
ciruit or other and the audio out. I wanted pro sound anyway, so I 
bought a USB soundcard thinking it would fix it.

Much to my chagrin, after quite some time of the new sound card working 
beautifully, I plugged it into a mixer at a venue I hadn't been at 
before, and the hum was back. Afterwards the ArsTechnica forum suggested 
that some mixers might have a ground loop switch on the back, that might 
fix it. The isolator link looks promising, but I need 1/4" plugs.

Glad to see other people have these problems and it's not just some 
weird mojo/vibes I exude ;)

-Ian

Mike Hord wrote:

>I bought a new Dell Inspiron 9100 a few weeks ago, and now I'm
>noticing a distinct hum in the background when I hook up the
>audio to my home stereo.
>
>Naturally, unplugging the wall adapter (or using a "cheater", which
>only has the hot and neutral but no ground pin) eliminates the hum.
>
>This worries me.  I fear that it represents shoddy isolation between
>the laptop and the wall, i.e., that one of the 3 connectors (inner
>sleeve, outer sleeve, and center pin) is a direct line to my house's
>ground.
>
>Part of what I like about a laptop is that isolation; it represents a
>little bit more assurance that in a thunderstorm, I may lose the
>power supply, but the computer itself won't be hurt.  Not to
>mention that since my house is a 30+ year old mobile home, the
>grounding setup is a little, ah, dodgy.
>
>Any comments?  Anyone else notice anything similar?
>
>Mike H.
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