Defender 110, 200 tdi Clunk when coming off the gas at speed.

Anyone has any suggestions where I should start looking at first. When driving on the motorway between 55 to 70 mph when I take my foot off the accelerator a clunking/grinding mechanical noise appears. It immediately goes when putting the power back on. All seems normal otherwise. Only seems to happen at speed so I don't think its the A frame ball joint which would happen all the time when in traffic.

I'd look at prop shafts first

Or could be gearbox output shaft?

Check your gearboxes and diffs for backlash and oil levels.