OK, here's the history on the 1984 300DT. approximately 5000 miles ago, I installed a valve stem seal kit and installed new valve springs and then performed a valve adjustment while the engine was cold. A "metallic tick tick tick" sound was heard afterwards.

So, I took the car to a mercedes factory trained mechanic who suggested that I readjust the valves with the engine hot. I did what he said and the noise disappeared. The engine has been fine until last evening when I opened the hood and listened to the engine at idle. It sounds as though the "metallic tick tick tick sound" is back though not as prominent. Is it possible for one or more valves to go out of adjustment after only 5000 miles?

If you didn't properly tighten them back up after adjustment they could. And its an easy mistake to make. I would at least measure the valve gaps and make sure.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
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Section 609 MVAC Certified
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"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

The cause of noise is often elusive. I have a loud tick tick that sounds like it is coming from under the Hood somewhere but infect is coming from something loose in the Exhaust system.
So I would get a Mechanics Stethoscope or as Stick or something and use it to listen around and see if you can determine exactly where the sound is coming from.

Also if you adjusted the Valves Hot you ought to listen for the sound and idle when the Engine is hot.

In any even it is not too much trouble to recheck the Valve clearances.

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84 300D, 82 Volvo 244Gl Diesel

Yep...start a process of elimination. (keep notes) Eventually you will either find the source....or eliminate everything else that might be the source.

Assuming of course....the particular ticking isn't a common or usual sound. Which I do not believe is the case. Injectors can intermittently make some rather disturbing sounds. I've never had in injector start nailing in my Benzes yet....but I did have a Bosch Injector in my truck do it. And they are similar in design.

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Proud owner of ....
1971 280SE W108
1979 300SD W116
1983 300D W123
1975 Ironhead Sportster chopper
1987 GMC 3/4 ton 4X4 Diesel
1989 Honda Civic (Heavily modified)
---------------------
Section 609 MVAC Certified
---------------------
"He who fights with monsters might take care lest he thereby become a monster. And if you gaze for long into an abyss, the abyss gazes also into you." - Friedrich Nietzsche

I guess I'll start the process of elimination.....I'm thinking of trying an Italian tune-up first, the car is rarely driven on the highway so it needs to be blown out anyway. After that, the mechanic's stethoscope is a great idea though sometimes it is difficult for me to determine where the sound is coming from. Diesels make a lot of racket, don't they?

The vacuum pump can also make ticking noises. The stethoscope might be in order if you don't have a good ear for this kind of thing.

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'95 E320 Wagon my favorite road car. '99 E300D wolf in sheeps body, '87 300D Sportline suspension, '79 300TD w/ 617.952 engine at 367,750 and counting!

OK, a pattern has emerged; the engine sounds normal when cold, it only tick tick ticks when hot. I'm going to get the stethoscope out to see if I can figure out what is going on.......