OK. Here's what I know.
First off, the environment inside a computer is absolutely terrible for an analog signal.
If you are hearing a rapid deet-deet-deet-deet (pause, and repeat) most likely you are hearing interference from the voltage converters in the machine.
one of the best notebook lines I have found for analog output of audio are MacBook Pro's (audiophile Apple notebook users, stop reading here, if you don't want me to spoil things for you...)
The only quibble I have ever had with apple audio output from their notebooks is that there is a slight pop as any sound starts playing. (I never noticed it till someone mentioned it, and now it is impossible for me to NOT hear it...) this is believed to be the audio output hardware being powered up. It is possible to prevent this from happening by keeping a quicktime window open, thus keeping the audio hardware active.
More than likely, most of the interference will go away as soon as you start running on battery power. I can also state for the record that the analog out on a Sound Blaster Audigy 2 ZS in a laptop cardbus slot also has this problem. I cannot be certain that a USB device will solve the problem, but your best bet would be to get a USB interface that draws power from a wall-wart rather than drawing off the USB bus alone.