Playing my Epi SG today I noticed that when using the neck pickup i could hear a static sort of sound when playing. Doing a bit of experimenting I found that the sound only occurred when strings were vibrating, upon muting the strings the sound instantly stopped and all I got was the standard low level hum. I also noticed that the sound was louder the thicker the string, so more pronounced with my low E and slightly softer on each progressive string. It is not happening with my bridge pickup. Any ideas what's going on?