Hello Andy That sounds just like mine, they are both running perfectly. Man and machine in harmony.
I always considered the original style of rocker adjusting screw to be a bit p**s poor. As the valve and rocker operate, the contact area of the adjusting screw becomes smaller, tending to wear a depression in the top of the valve, which give erroneous clearances with the feelers. As the noise is louder from the rockerbox, this is where I would start. A set of Mushroom headed adjusters may be the answer.
If you think it is too far advanced, close the points gap a bit, this will retard the ignition as the mag then has to rotate slightly further before the points open. Quick and easy as a test without having to mess with that ATD bolt.
In the meantime it still runs as is, and whatever is knackered is screwed anyway so use it til it stops. Then you get the answer. That's Zen and the art of......
Keep Spannering
Swarfy.