Diagnose my noise please!

Hello!

I’d appreciate some help diagnosing a noise and subsequent fix. My vehicle is a 92 Defender 90 hard top on 73,000 miles.

Here’s what I can tell you:

this is is a transmission based noise; engine is fine when vehicle is stationary/clutch disengaged
The noise varies with speed
It is not present when the engine is in gear and driving along normally
It typically appears when de-clutching to approach a junction
It is sometimes possible to make it appear if freewheeling down a hill with clutch disengaged
It goes thunka-thunka-thunka-THUNKA-THUNKA——THUNKA———THUNKA getting louder as vehicle slows. There is accompanying vibration.
It feels like it is coming from underneath the drivers left foot area

It’s clearly a rotating part and my guess is something to do with the propshaft.

all ideas and fixes welcome. This was an occasional noise over the last year or two but is getting frequent. I only drive the Landy in the winter and it came out of hibernation a couple of weeks ago and it’s pretty noticeable

Ujs

Front of gearbox UJ is most likely, get it sorted quick as if making that much noise it may well let go completely allowing prop to thrash about causing more damage.
May well need a complete new prop.

Thank you. Looks like I need to spend some time under the vehicle...