I don't think it is valves. I adjusted them about 8000 mi ago. Timming chain strech was a little over 2 deg. I will go through them again. By "snow studs on a dry road sound" I mean a scratching like a car going past at 40mph not snap and pop 5mph

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1980 300TD with 1985 300TD Turbo motor and 240D 4 speed.