I have a Tacoma ABG, it's not the fanciest ABG, but it sure sounds great when plugged in. It has the Fishman Classic 4 preamp... I'm wondering if I have some sort of ground hum, I notice a hum when it's plugged in... the only way it goes away is if I touch the metal sleeve of the cable. Is this just the nature of this thing or is this something I can fix? Thanks.