Head Fin Ringing

So the ringing noise is a frequency vibration like a tuning fork and not an amplification of internal engine noise? My hearing has never been the best and I can't say as I've ever noticed anything other than the usual engine rumblings on any type of 2 or 4 stroke engine.

Yeah, It seems to be more of a resonance thing.
Old Ossa 2 strokes had very long fins and the rubber dampers between the fins tended to fall out after some time. Then they rattled loudly and if you continued to use them without replacing the rubbers, the fin would crack and fall off.

I heard the ring years ago on my Mk3. I found the cause was slight preignition probably caused by the removal of leaded fuel. I retarded the ignition by a degree or so and the ringing disappeared.