[8210GUY]

It sounds like the connection itself, or worst case the speakers, by the fact it works fine apart from those speakers suggests that all is well with your card etc, I would tend to look at the plug on the problem speakers, is it the same size as the other one ? are you sure it's plugged in firmly (and in the right socket) ? and can you plug them into anything else to see if they sound fine from there ?
the times I've had noise like that is due to the connection not quite sitting right, see if that helps.

 

[8210GUY]

If those cables work ok in another system then I doubt it's them, and TBH the only thing left when you reach that stage is the plug itself, take a look at the 2 jack plugs involved, see if one is bigger or has more ribs etc than the other, it's as though the problem plug is earthing somewhere when plugged into the soundcard, see how each of the jacks look when plugged in, I'm betting there is some difference albeit a minor one that is causing this, and if the plugs look identical then plug in the problem jack and set something playing, then ever so slightly take it out, NOT pull it right out, just a tiny fraction (1 or 2 mm say), see if the hum stops then.