Had my monitors for a while, then all of a sudden im hearing a faint thud like someone is banging on the wall. Then i notice its coming from my sub, which i have all balanced cables hooked up. Then it starts to get louder and louder and start to become a wobbly thud. It even continues when all of the monitors are turned off and disconnected. What is the cause of this, i've searched through all of the questions and haven't found my answer?