Thanks for all your input.
I seem to have located the sound now. It comes from the MSD box and the coil. Both sit on the passenger side firewall next to the engine and the buzzing tick is transferred through the wall into the cabin.
That explains why I can hear it with the engine door in place and barely when the door is off.
Apparently very normal for analog MSD boxes to make this quite loud noise.
It could be the time for me to update to the much spoken for Pantera Electronics digital unit
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