Only gone when you grab a connector or knob? and Still there if you only thouch the strings?
Every guitar should be deadquitte when you touch the strings, if that is not the case your guitar is not grounded good enough. Or this means: the strings are not grounded, which should be.
I had the same with a Epiphone Electro-Acoustic guitar of mine, the cheap Fishman built-in preamp was not grounded good enough. The noise only went away while grabbing the outputjack on the guitar. But I fixed this allready.