I have tracked it down to the power supply. The effect is there with the power supply on and connected to the server but with the server off. The effect goes away if the power supply is not connected to the server or it is off. I have noticed that the bubba is still on when it is off, e.g. it still powers the USB port. So I have no way of determining whether it is the power circuits within the bubba or the power brick that is causing the problem. Disconnecting everything from the USB port doesn't help.
What would be your advice in this instance?
