1985 300D 300K miles runs great no problems, reasonable pickup for a W123 diesel, nice and smooth...EXCEPT...

There's a strange screeching sound when engine is under load (heavy acceleration, going up a hill on the highway, etc.). It usually goes away on its own, if I change my speed a bit, or if I ask for more or less performance:
https://photos.app.goo.gl/xw6Rc5Do7QEZGs4V6 (4 sec vid)

I removed all the belts and drove it around the block and still got the noise so that eliminates the fan, fan clutch, water pump, and all the accessories.

I am able to get the noise when in park but that's difficult to do, it lasts for only a second, and it seems to happen only when the engine is warmed up:
https://photos.app.goo.gl/qyWoLKzksL2dQeEM8 (3 sec vid)

so that eliminates the driveshaft and friends.

So what's left? Could it be the turbo? Something internal? Is it serious? Can any issue with the timing chain cause this? I'm baffled.

John