Most diesels I've listened to have varying sounds at the lower rpms. I've read some clatter is attributed to the old style injection pumps. That could be the clatter mine makes at a certain rpm. It quiets down below and above.

I can't see what would cause a shrinking gap unless the valves themselves, or the seats, wear down. Any other part would expand the gap due to friction I would think. I guess that's another reason diesel engines last so long even if the valves have never been touched.