my 1984 300d with original engine, with 279k on it produces a hell of a nailing sound. if the car is not started for 2 days, you do not hear it until the engine is warmed up. i just had a recent oil chg and the independant mechanic that i went to (not my original.) once the engine is warmed up it sounds like a guy is under the hood hitting the engine with a hammer. i run the reg diesel fuel..use power service regularly. i have plenty of power....timing chain shows no stretch. what do you think b4 i junk this thinking i need a new engine?