My mic is unplugged, and how thick is a thick cable? I think it is a mobo malfunction.
The mic is unplugged but is the input channel MUTED in the sound manager ?? Even without a Mic plugged in the input can pick up distortions and amplify them as if they were coming in from a mic so if not using that channel it should always be muted in the sound manager unless in use !
By thick he means well shielded to prevent outside sources of radio interference being picked up and amplified by the wiring causing distortions to the sounds (if using non shielded wiring it can pick up ambient noises from things like your fans etc. and amplify them thinking they are playable sounds.
^+1. This kept happening on my laptop and it took a good 10-15 minutes to find the problem. After disabling the Mic input (I don't use it) I have not had this problem. Granted, this was not as bad as the OPs problem (only happened randomly).