Welcome to the club, but just because it was built in 1975, doesn't make it old.
Although you may be aware of this, I'll mention it anyway. Before I tore apart my first transmission, I always thought the gears were not engaged until they were actually selected. I thought, "first gear" could be stripped and 2nd-4th could be fine. But transmissions are not made like that. In fact, all gears are always engaged always with their mate (with the exception of the reverse gear). All gears teeth are always rotating with their mates rotating teeth. The gear selector simply allows one gear to no longer slip on the main shaft -the selection of a gear connects it's rotation to the shaft. 4th gear, is simply locking the input shaft to the output shaft, which isn't to say that no other gears are involved -they are all still spinning.
Trying to guess what noise your transmission is making, or why, is just a guess. Is it full of oil? Did you change the oil in the transmission? Are you sure it's not the universal? Does moving the selector around slightly change the noise it's making in any gear? Does it shift smoothly?
All said, if you don't have the cover on, or carpet and/or insulation, these transmissions are not exactly silent. 1st and reverse are noisiest. 2nd-4th noise should be overcome by the traffic noise, wind noise, road noise and any squeaks/squeals/rattles the old car may have. Any sudden stop, say from a broken tooth jammed inside that transmission can make for a very bad day.
This group can help you fix anything on your TR6 but you need to be careful.