Definitive Supercube Reference makes clicking noise

I hope someone can help me. I have a Supercube Reference subwoofer I bought in 2007. Recently the subwoofer started making loud short "click" noise every 30-60 minutes. It clicks when the sub is ON with input signal or OFF when there is no signal or even no LFE cable connected. It clicks even when the volume knob is the minimum volume position. The sub is not turning on or off when it clicks. The click noise is a little different then the low frequency POP noise when the sub is turning on. Do you have any idea how it can be fixed and what is the root cause of the problem?