Does it happen on all channels? You mention that when you pluck the strings you hear it, but when you mute them it goes away. What happens if you don't touch the strings or any metal part on your guitar at all? If the noise appears then, but goes away when you touch the strings, I would thing that is a ground problem on your guitar, or the input of the amp. If the noise happens only one channel, its probably a pre-amp. Test that first.