Humming noise in built-in subwoofer of Polk Audio RTi 100.

Hi, I have a pair of RTi 100 tower speakers in great shape, both have built-in subwoofer amplifier system. One of the tower speaker is generating humming noise when it is switched to "Auto" that kicks in the subwoofer function. (Both the towers are hooked up to the AC outlet). Also, when I move the volume knob for increasing the sound, the humming noise alo increases. I called Polk Audio and they provided me with Canadian Ontario Technician's contact but it is mostly difficult to communicate and get info from him as he is travelling most of the times it looks like to me, plus I do not know if he provides with a warranty after repair of the amplifier circuit plate. The postal charge is too much to send this plate to Ontario from Newfoundland and one of the local technicians told me its all digital switching plate so has to be replaced as they are difficult to repair.
Now any one of you knows about this or have any experience, I do not want to junk them as they are absolutely wonderful speaker set.
Thank you in advance.