You are fortunate. On all of the Ct 200h that I have driven, particularly on dual carriageways and motorways, the engine noise has always been noticeable along with the road noise. Should the engine fail I feel sure that you would become aware immediately as your other instrumentation would alter. As for overtaking, the engine noise from increased revs would be immediately noticeable. Maybe call in at your Lexus dealership and see if they are aware of a way to overcome this matter.