Your sound card. I don't have one but whatever sound can come through. Maybe the mother board or video card?
I've been having this cracking noise coming out of my 5.1 speakers I don't think they are blown because I switched one of the lines with two other speakers and the exact same cracking noise was coming out of them. There is also a cracking noise coming out of my regular computer speakers that I have not noticed before. Also on the side left and right speakers there is like a echo it doesn't sound like it used to.
I'm thinking a pc sound problem are maybe my receiver melted or something.