Hi Richard,
I am not sure that probabilities favor the electrics hypothesis.
Most electronics without moving parts remain quite silent unless they
are in the terminal stage of releasing their electronics, steam and smoke.
Have you ruled out air, coolant, or oil flowing through some constriction?
Carbon pile voltage regulators could make noise as the carbon heated,
but I think that species went extinct in the 1960s.
Best of Luck!
--------
Ira N224XS
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