Rumble in '81 300TD Turbo

I'm getting this odd rumbling from about 30 mph through 50 mph in my 300TD Turbo. It may have always done this since I got the car, but I haven't noticed it as much until lately because it's gotten cooler outside and I've
been keeping the windows up and the roof closed.
Along with it is a sort of 'pulsing' vibration. It's similar to what you would feel and hear if you were driving something with about 400 HP with headers and turbo mufflers. It has plenty of power and I've changed the fuel filters.
The actual ride is smooth, but there's this pesky vibration.
I want to think it's nothing more than the prop shaft bearing, but now and then I get the sense it's coming from the engine or transmission. It doesn't seem to get any worse or better when accelerating or decelerating, but seems to be worse when crusing at around 50 mph.
It's very noticeable when you accelerate to your desired speed and then back off the throttle to maintain that speed.
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I would think may be: Air box may be loose Wheel bearings Wheel bushings
WDTY?

I've
fuel
doesn't
seems
back
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Perhaps wheel bearings, but I'm told (probably shouldn't believe this) that the wheel bearings were checked and repacked last year when the pads and calipers were changed. It could be a few things and I'm eventually going to change the wheel bearings and axles anyway so I know for sure the stuff is good, but was just wondering if anyone had seen a similar issue.

300TD
and
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that
to
Could be the flex disks or even engine mounts
The prop shaft bearing is also a possible cause - it however causes a harmonic vibration throughout the car.
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Wheel bearing is easy to check... just jack up the car and put it on jack stand... then grab the hold of the wheel 180 degree and rock it to see if any play, if yes, then it is shot... you will know when you compare it to the other side.
Rear wheel bearing will clunk...usually the right side is shot. Do not replace the good side... it will never go bad... US road is just worse on the shoulder side. Front wheel bearing would roar when cornering.
I would suspect the flex disc that connects the transmission to the driveshaft.
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Think I found the source, but correct me if I'm wrong...
When I grab the prop shaft near the intermediate bearing, it moves VERY easily. Never messed with one of these, but based on other cars I've seen, the rubber support should be pretty stiff like an engine mount. This thing moves all over the place a good half inch.

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that would cause some problems, alright!
You have found the problem.
Bernard
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