It sounds like amp noise. Have seen some amps do it on their own (lesser quality amps), and have seen poor quality interconnects to the amps induce noise that becomes amplified. Your amps are probably music sensing which is why you don't notice the noise from time to time, until the noise gets picked up and the amp turns on to amplify it. Good luck. Why are you bridging you center channel? Be careful when you bridge, I had a manufacturer recommend bridging the center in their system until they realized 125watts bridged to 250 into 4ohms was too much for their speakers (500+) and they were blowing speakers.