It could be a couple different things. First of all, you could have a bad ground in the guitar somewhere. If it's pretty new and you haven't messed with the 'guts', that's probably unlikely.

The more likely possibility is electrical interference (though this is more common with single coil pickups). Stage lights, fluorescent lights, etc. can cause annoying hissing/feedback. Try moving around the room and see if the noise decreases... If so, stay there and don't move! :D