I have a 1982 240D with 235K on it. The car runs very well with good power and fuel consumption. Lately there has been a 'pinging' noise while driving. It sounds like a gas engine with timing that is too far adavanced although not as loud. The pinging does not get worse when the engine is under load, but will be apparent during steady a cruise. I have owned the car for about two years and I have no service history for it other than the general maintenance that I have done. Does any one have any ideas as to what this noise could be?