CS:S Sound Problem

[aplefka]

Well, last night I noticed a strange humming noise in the background while I was playing. I thought it was the headset I was using, because it was a really cheap one, but then I switched over to the speakers and it was there too. It's just a constant hum that goes while I'm playing the game.

 

[MisterChief]

Are you using a built-in sound card?