It is not just the usual low level background 'hum' and not a feedback squeal. I guess I could try recording it later?
OK so I cut about 0.5mm of the insulation from around the base of the pin and hey presto...noise is gone!
I second the photo request @CESIL and just out of interest, may I ask what brought you to that conclusion?
I've experienced the hum, 'sometimes' if you don't have the manufacturers software installed the sound card won't change to the correct output mode. it will treat every device on the audio out as "headphone" but with the manufacturers software, Realtek in my case, you can see it switch the output to "line". When it does this the ground loop issue goes away.
I've seen this with sigmatel as well.
This issue has hit me more recently as we've moved to windows 7 that normally has the right sound drivers in already. So we didn't bother with the manufacturers version. Whereas on XP we had to install it.
Sorry for the slow reply folks, but I was not at work last week...
For those who wanted to know what the fix was I have produced a natty illustration which will show it better than a photo...
@Koldov...I found that pushing the plug very firmly up against the laptop socket stopped the noise and that suggested that that there may be a contact problem...
Ah I can see now what the earlier description meant (I was thinking you were stripping insulation from the cable, hence the wut?). Luckily, your natty little illustration explains things very clearly (a picture paints a thousand words...) :) Thank you.