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Lifter noise is VERY common on the OM60x engines. It will be related to engine speed, but NOT engine load. Can change with temp and be intermittent. Use Mobil-1 15W-50 for a few oil changes and see if that helps. On my sister's 1987 300D, it cured it in days - she was pretty happy about that. When the lifter(s) get bad enough, synthetic won't help, you'll need to replace the collapsed lifter.
Belt shock noise doesn't change with RPM but may go away at certain RPM's as the vibrations & harmonics of the belt change with engine vibration, etc. This is simple to test - at idle, use a broom handle to press *sideways* against the belt shock. If it's the source of the noise, it will get silent when you press on it. BT, DT. Only use an OE (Genuine Mercedes) belt shock, yes at twice the cost, unless you really enjoy changing aftermarket ones every 20kmi.
A hard noise that gets louder under load could be "nailing", which is sort of like pinging/knocking on a gas engine. Diesel Purge should help or cure it, if not, the injectors needs to come out for a pop test (pressure & spray pattern).
My money is on the lifters - that is the most common, and luckily it's an easy cure most of the time. Don't use SynTec, Rotella, or Delo "synthetics" - use Mobil, Amsoil, or RedLine (and don't use the 0W-40 stuff either).
HTH,
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