Could be the pickup selector ground, or some sort of shielding issue, among other things, but those are the most common I've seen. You might also have a noisy switch - some of the lower-end toggles are like that sometimes. If it's the selector ground, it's a pretty easy fix (just resolder it and it should be fine). If it's a shielding issue, it's a little more time-consuming (getting shielding tape, putting it in properly, and re-grounding) but not hard. If it's the toggle, it should probably be replaced.