This may sound strange but try it. This will tell you if the crackle is generated by the computer- PSU operation.
Open the bookmarks page of a any browser you use. Scroll rapidly up and down the lists while listening to the earphones with no audio source playing. Doe the crackle shift rapidly on and off while you are scrolling? Mine does on the Compac I have on the office desk. The crackling noise is quite intermittent during this action. When stopped or slowed, the crackling is more constant.
The only solution for a laptop that I can think of since putting ferrite beads on the fan DC supply wiring is not possible, is to use a USB audio interface if the noise is too annoying.
