Here are the symptoms then...

1983 300td: Car has unknown history - probably >150K on it.
Differential was leaking oil pretty badly when
I bought it. Level was low and I ended up replacing
the input pinion oil seal as that is where it was leaking from & replacing oil.

I get a rotating & groaong noise from the rear when I littly accelerate.
It becomes noticlible at about 35mph
It appears to reduce if I tip in & accelerate harder
but it could just be being masked by the general increase in noise.
It dissapears if I lift off from the accelerator pedal.
It appears to get worse as the car warms up i.e. after 5-10 miles.

I've put new brakes on it and checked parking brake.
The car has been inspected/tested and wheel bearing were presumambly okay.

The obvious choice would be a worn diff but how can I tell whether its the diff, the drive shafts or bearings?

Steve

Noises and vibrations are near impossible to diagnose over the internet. In your case, however, your description is pretty good.

From your description, since it is load dependent, sounds like the center section. As long as it is filled with lubricant, I would turn up the radio and forget about it.

If your mental makeup prevents you from following that advice, your best bet would be a used center section from the salvage yard. These units have a great track record and as long as they keep grease in them they will seemingly last forever.

Make sure you get the same ratio.

Good luck,

the diff was a leaker & a howler when I got it and is now just a howler - Tightening up the pinion nut was a bit hit & miss I remember
steve

Often times a very active leak from a seal indicates trouble deeper in.

I can not hear the noise, however a failed inner or outer pinion bearing will give you a growling noise that changes on accell/decell.

Side bearings will always growl with a slight change doing a figure 8..

Axle bearings will growl and be more, or less pronounced when doing figure 8's. or shifting load going down the road..

I am betting a bad outer pinion bearing in your case. (the seal leak is my clue for outer)

Don't confuse the growl witha howl/whine noise. Thats ring/pinion noise...

If the pinion gear is sloppy enough it could cause a irregular wear pattern on ring and pinion and you could wind up with a whine. That pattern should be checked and tested with some blue grease applied to the teeth.

__________________

79 240D my current toy
42 years a Diesel addict

240D sold
250SE sold
220D sold
280C sold
280S sold
300D (2) sold
300CD sold
300DT sold
300SD sold
380SL sold

is there any risk of catastrophic & dangerous failure with the diff ? Can I just run it til I get fed up with it?

Steve