Sounds like your card is picking up electrical noise or rather electro magnetic interference. This is caused by induction and can be a sign of a poorly designed sound card. Your speakers may lack appropriate shielding and could be picking up motor noise from the hard drive. Your audio cables may lack adequate shielding. You could even have a power cable resting too close to your sound card.
There are literally dozens of possible things or combination of things that can lead to interference with your sound system. The best way to ensure good quality sound is by installing and using a good quality sound system.