Alot of transmissions make a rattle or gear chatter sound under load at certain engine RPMs from harmonics, if its a constant speed or RPM related rattle it could be a chipped tooth even from age.
Only way to see what needs replaced would be to tear down the trans and visually inspect all of the bearings for wear, replace and readust preload or endplay accordingly.
it could be anything though. usually once a trans is opened up theres more problems found than you would expect caused by others that started long ago. Post a video of the noise if you can.