Your reply is most helpful. I’ve rebuilt the engine, replacing the crank, clutch housing, plates, etc (amongst others). I was pretty careful about measuring the end float on the sliding gear. So I was surprised when I heard it rattle and whine. Initially I thought the whine was from the primary gears, so I bought a new set from a German website. When they arrived, I was a little alarmed at the poor quality of the machining/casting, but fitted them anyway – just as noisy as before. So, I bought a more expensive set from the MZ shop. These were much better quality, but after fitting them, the whine was unchanged.
After reading your message, I think I might just ignore the noises, and keep on riding it.