I just replaced a sub in my two subwoofer setup(I now have two 10 inch, 120 watt subs)and when I plugged it in, I hear a " switching on/off noise" in the new sub. I don't hear this coming from my other sub and I did not have any cables hooked into it. What could this be? Has anyone else experienced this? Any suggestions? Please Help, I've been dying to hear my new set up.