I had the same noise on my old engine, ('78 no turbo) it was bad when I got the car and knew that it would eventually cause trouble but the car was relatively cheap to start with. the noise was a rapping sound, always present, and it would change pitch when you let off the gas. eventually the car just quit one day and would not restart. I had already been searching for another motor, and found a car with a decent one for $600. That motor is now in the car. The oil pan of the old motor was filled with the remains of a piston.
Soooooo, the newer motor also has this sound, but it is much quieter, you have to listen for it. seems to be in the front of the engine. Always there, a can of diesel purge didn't help. My question is: how hard was it to change the one piston? I like to think I have OK mechanic skills, but the only motor I have ever torn down was a VW aircooled, I doubt this will be that easy. Will I be able to remove the head and oil pan and then unbolt the rod on the offending cylinder and remove just that one to replace it? Does not seem like it would be so hard. will it be?
thanks for all opinions, Andy

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andy t
'78 300d
'95 volvo 850, wagon
'86 300sdl - engine out, maybe I'll have it rolling by June

whole bunch o' bicycles