Mercedes Benz repair: 1981 Mercedes Benz 300D, mercedes benz 300d, benz 300d

Question
My 1981 Mercedes Benz 300D is making a very loud put putting noise intermittently. It sounds like a bad gas knocking but it only happens when I accelerate. I usually have to drive some distance over about 40 for it to start. Running up to the grocery store in the neighborhood does not usually cause it but it almost always starts when  I go to work on the expressway. It seems to come from the middle to back of the car, not the front. My mechanic has checked the exhaust system and the drive shaft but nothing wrong there. I know the fuel injection is in the front but is there any part of that that could somehow cause this noise nearer the back of the vehicle? Any other ideas welcome! Please help.Thank you.

Answer
When was your last valve adjustment?

There should be only three things that could fit your symptoms.
#1. Bad exhaust system = resonator and/or muffler coming apart inside or rusted pipe preparing to break.
#2. Turbo failing.
#3. Burnt exhaust valve.