Are you practicing near ANY other electronic devices at all?? and if you are does your guitar have single pickups instead of humbuckers? this is what happens to me, if i plug in my strat (fat strat s s h setup) and i flick on to the neck single i get hum all the time seeing how i have so many electronic do dads in my house here. But other than that play with your gain settings some more till it goes away.