It is due to your pickups being hotter then probably the ones you are use to. It happened to me when I put in a Seymour Duncan JB, it is way hotter then the standard jackson and there is a lot of hiss. If cables don't fix it, then maybe a noise gate (although I doubt it). If neither of those work, then fuck it. Who cares if it hisses when not playing? As long as it doesn't hiss while you are playing then don't worry about it.

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