subwoofer hum


I have searched these threads and still at a loss to why my sub is humming all of a sudden.  Here's the story.  I have a Denon AVR487 receiver and sub-woofer.  I wanted to control the volume of my indoor speakers when I had the outside speakers on so I installed an audio switch from radio shack.  It turns off my surround speakers so its not too loud inside when I have it on outside.  Once I did this and not sure if this is coincidence or not my sub hums now really loud.  I have tried a new cable, removed the switch, plugged the power into a separate circuit and it still hums.  I talked to Denon and they sent me a Boston Acoustics sub, tried that and it still hums.  I am dumbfounded why this is making this humming all of a sudden.  I even unplugged all the speakers from the receiver and it still hums.  This may help; I unplugged the sub cable from the receiver and the hum stops but if I touch the end of the plug it makes the hum noise.  Any help would be greatly appreciated.  Thanks.