Weird transmission noise
 

Anyone help on this? My '61 T5 S90 (type 741 trans) makes a groaning noise with wheel rotation when going up a steep incline at walking pace (the steeper the incline, and the hotter the transmission, the louder the groan!). Have changed wheels/tyres and axle bearings, but still there.
No noise at higher than walking pace, or if no incline, or reversing up incline. I'm stumped!

Possibly pinion gear bearing is worn, shifting under load
> noise due to ring and pinion out of alignment??