Recently I can hear a slight rumbling from the gearbox/dif. It comes around 80 kph and on a road with slight ups and downs the noise changes as the car goes light or heavy in the dips. My feeling it is either dif or output shaft bearing - THOUGHTS PLEASE

Hit it wiv a big 'ammer?

Any sign of free play at the outer edge of the drums (or the discs, if it's been "LHM'd")?

I'd suspect differential bearings first, mind you.
Might be worth draining the gearbox oil and having a look to see of there's more than the usual collection of 'whiskers' on the plug, as there'd probably be flakes if a differential bearing was beginning to let go.
They're standard 30207 taper rollers, iirc.

The output shaft bearings are not lubricated by gearbox oil, instead they're the 'greased for life' type and are located outboard of the output shaft seal in any case.

ken

The play you feel on the drum or disc is usually the planet/satelite gears.

Lift one front wheel off the ground ( jack underneath bolts securing the steering rack ) Start the engine, out it in 4th gear and gently let go of the clutch. Take the speed to around 80 clicks. Quickly place the gear box in neutral and turn the engine off, then listern to noise from the diff and out put hub. Best is 2 people, one driving one looking

Swap to the other side, same story. Be sure that the car is secure and not jumping off the jack. Maybe block one rear wheel with bricks

I drained the oil about 2 weeks ago and nothing ab-normal there. I will try the up in the air test tomorrow if weather ok. THANKS

Viking,
I didn't mean rotational play, as that's always present, not just because of backlash in the sun and planet gears, but also because of the sliding fit of the output shaft in the differential.

On the other hand, a worn output shaft bearing can have enough 'rocking' motion for a brake disc to chafe against the caliper, although most folk realise that something's wrong before it gets as bad as that...

ken