I have a 1993 Mercedes 308d. It always used to be fairly quiet, but recently the engine has started to sound quite noisy. Its OK when started then after it gets warm you notice when you come to a standstill that its quite noisy. It like a slight tapping chuggy sound.
I have had it checked by a Mercedes specialist who could find nothing external. I've had the injectors checked but just a small blockage which was unblocked but has made no difference. I have put injector cleaner in the diesel, new fuel and oil filter and oil change ( all of which I usually keep on top of anyway )...but no difference.
I'm out of ideas and the mechanic at the Mercedes specialist said he would have to take the rocker cover off and see if theres anything in the engine thats worn or broken, though when its not making the noise, the engine sounds sweet and it runs OK.