This can be caused by the “input switch” on the unit’s rear panel not being fully engaged in the proper position; make sure that the “input switch” is fully engaged in the proper position (RCA or XLR). Also, make sure that only one input connector is used at a time, i.e., only the RCA or the XLR. They should not be used simultaneously. Explanation: The “input switch” is actually a ground switch, not an input selection switch. By using the switch only to change the ground connection, the signal connections never have to pass through a set of switch contacts. This allows for superior audio performance and enhanced long-term reliability.