My 93 300D 2.5T has been making a clacking sound at around 1400-1500 RPM. It sounds like a valve clanking like sound. It only makes the sound when in gear. If I rev the motor in park - no sound. It is worse when cold, but doesn't go away when warm, just less pronounced. It is only audible at the specific RPM range. It sounds like it's the top back of the motor from the drivers seat but who knows if that is accurate. Anyone have any ideas what's going on. The motor has 125K on it. Power and shifting is normal and fine.

Could be a lifter starting to go bad.
When was the oil changed? what type & weight?

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Check also for leaks around the pre-chambers.

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Oil was changed 2 weeks ago. Using Rot T, 15W-40. Has been doing this for several months over several oil changes. I've used this oil for the last 50K since I've owned it. Changed every 5K religously.

No leaks around pre chambers visible from the topside.

Also remember, this seems to only be happening when in gear and moving. Not when in park or neutral. Seems related to trans, but as I said sounds like the motor. It also has the sound of a loose bracket or something similar. Not saying it is, just trying to describe the sound. Kind of a rattle if you will. No idea what to do other than wait for it to get worse and hope it isn't catestrophic

you may have problems with the belt tensioner bearing or the idler bearing.

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Could also be a motor mount issue.

You can try jacking the motor with a block of wood ONLY an inch, leaving the jack & wood and see if the noise goes away in revese at that RPM

BE very careful & only raise the motor slightly!

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13mm test. Apparently you can run a 13mm dowel (head of 3/8 extension?) from motor mount to motor mount as a way to see if your mount has collapsed. Never tried it but it was posted on okie benz a bit back.

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If the above suggestions are not fruitful, maybe a check of your drive line is in order, flex joints etc. Last suggestion (I hope not) torque converter attachment bolts loose or worse, the trans pump drive on the converter has one side cracked or broken.

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