Probably worn cups, axle cones and bearing balls.
If BB threading is made to a modern (BSA, or even Italian) standard, I'd replace the whole thing with some cartridge bearings and new cranks.

If you don't want to go that route, open it up, clean it, inspect for wear. Bearing ball change is certain IMO, but check for cups and axle cones for wear - they might need replacement as well. Also, new pins for holding cranks in place should be sourced before mounting it all back. As well as some grease.