My subwoofer has suddenly taken to intermittently humming loudly, but other than that, it appears to work fine. The humming initially started whenever my 5-year-old son would jump from the sofa to the floor (something he’s inclined to do despite his parents’ admonitions). The subwoofer is located behind the sofa, and it appeared that mechanical vibration would set it off. I can always make it stop humming by simply touching with my finger the metal where the RCA signal line plugs into the back of the subwoofer. Now the problem has grown worse, with the subwoofer suddenly humming without apparent provocation (it’s loud enough to wake us up at night). Again, I can always make it immediately stop humming by simply touching with my finger the metal contact of the RCA signal line. The subwoofer is a Yamaha YST-SW105 Active Servo Processing Subwoofer System. It’s about 6 months old. Any suggestions?