I am confused.

Title states "under way"....the sarcastic side me would suggest to go "overway" instead. LOL.

The description then states alot of sitting still. start in one area. get the noise to reproduce. When it does, put the trans in neutral, allow rpms to raise and lower. Does the pitch change with RPM? Then the squealing is coming from an accessory or a motor component.

use a stick or rod. Touch one end to each accessory and the other end put to your temple or skull. The noise will radiate to your ear and be loudest when you are closest to the culprit.

be mindfull of the belts and cooling fans (the alternator has a fan as well).

If considering the power steering pump, does turning the steering back and forth effect the sound? (not just by effecting rpm as the pump creates load) Does the sound get worse?

best of luck.

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83 300TD (need rear wiper assembly dead or alive)
84 300SD Daily driver
85 300TD almost 400k miles and driven daily.
98 E300D *sold
86 300SDL *sold and made flawless 10 hour journey to new home.