Occasionally, often after just getting into the car, I get two or three loud thumping noises from the Columbus speakers. This happens before I even put the key in the ignition and so before the unit is turned on. It sound rather like the sort of noise you might get if you touched a battery across the wires of a bass speaker. Other than that there are no other symptoms I've noticed and the unit seems to work fine. As far as I can tell it has never happened when the unit is tur