a control unit 11 responds in ordinary course to external stimuli . in addition , the control unit 11 further processes the external stimuli to characterize and otherwise seek to identify certain aspects and attributes . unique audible signals that correspond to varying results of such analysis are then provided . these audible signals can readily facilitate ease and accuracy when diagnosing the cause of a particular operational issue . in one embodiment the diagnostic audible signals are selectively mutable without concurrently muting other audible signals as correspond to ordinary operation of the control unit 11 .