My Computer speakers make a noise?

My computer speakers make a static noise when I am not even using them. When I am playing music or anything of that nature, they sound O.K. The only way I can stop it at all is to totally mute it in the system tray. (the little speaker). I am thinking of changing my sound card as well. I am using an HP 5995 computer (practically new) 2.1 gig P4, and a gig and a half of ram. Works great otherwise except for the static noise. It is very disconcerting when you are trying to concentrate on something. Could someone tell me the optimum sound card I could install on my computer, also.