|I don't know what this is but if you can only hear it through speakers or headphones (even with volume turned down) then it doesn't sound like HD trouble to me. If the HD is failing I would expect mechanical noises from within the vicinity of the HD drive.|
It could be some issue with the soundcard itself.
I have run into background noise which can be reduced to almost nothing by turning the software volume up high and the speaker rotary volume control down (or even vice versa). This can improve the signal to noise ratio, depending upon where the noise happens to be injecting. You might then need an in-line headphones volume control.
some other bloke...