Why do I have a crackling sound with my hd downloads?

Best thing you should do until you check that the problem is solved is BACK-UP all the important data there is nothing worse than losing everything. I suggest you re-install the DIVx player and see if the problem goes away, secondly it might be your hard drive is slowly dying and watching the movies means that its constantly spinning meaning it always under stress, then your web-surfing the hard drive is not in use so there is not sound.