1986 RR 2dr Manual gearbox problem

I've just bought and picked up (today) a 1986 Range Rover 2 door V8 with a 5spd LT77 gearbox. When the engine runs there's a noise like a rumbling bearing from the gearbox. It's more prominent in lower gears. When the clutch is depressed the noise disappears, which makes me think it's not the release bearing. Any ideas? If I find a replacement gearbox on ebay, are there differing ratios? If so, which is reckoned to be the best?