Dirty potentiometer. Make sure there is no input signal, and then rotate the pot back and forth all the way many many times. This will scrape off dust, oxidation, whatever. After 50-100 times it should be mostly if not all crackle free.
I actually tricked a friend with this one time. He had some PC speakers with a real cheap, small inline volume control on one of the wires. I told him to stand back and "watch the magic". I grasped the volume control in my hand, and rotated the knob back and forth real quick in a such a way that he couldn't see. After 30 seconds and some "alakazam" ********, I handed it back to him and said "all done". When he rotated the volume knob and there was no crackle, he looked at me and said "You son of a *****!". I really got him good with that, never told him how I did it.