Going back to when I was told that there is no advance/retard mechanism in the magnetos.
Web trawl tells me that Bendix 20/200 series are used on this engine.
They have impulse couplings. Which should drop out at 250rpm.
Two things.
1. Are the coupling pivots fitted with snap rings as they should now be? An AD relates.
2. Any chance that the start switch is malfunctioning and staying live?
Before I get my hat and coat, I can't help but focus on the fact that when the mags were replaced, the rpm at which noise occurs rose by a couple of hundred rpm.